Applied Zoology

Home , Acetabulum (morphology), Eucestoda

Animal Diversity- I (Non-Chordates)

Phylum Platyhelminthes

Ranjana Saxena Associate Professor, Department of Zoology, Dyal Singh College, University of Delhi Delhi – 110 007

e-mail: [email protected]

Contents:

PLATYHELMINTHES

DUGESIA (EUPLANARIA) Fasciola hepatica

SCHISTOSOMA OR SPLIT BODY

Schistosoma japonicum Diphyllobothrium latum Echinococcus granulosus

EVOLUTION OF PARASITISM IN HELMINTHES

PARASITIC ADAPTATION IN HELMINTHES CLASSIFICATION Class Turbellaria Class Monogenea Class Trematoda Class Cestoda

PLATYHELMINTHES

IN GREEK:PLATYS means FLAT; HELMINTHES means WORM The term platyhelminthes was first proposed by Gaugenbaur in 1859 and include all flatworms. They are soft bodied, unsegmented, dorsoventrally flattened worms having a bilateral symmetry, with organ grade of organization. Flatworms are acoelomate and triploblastic. The majority of these are parasitic. The free living forms are generally aquatic, either marine or fresh water. Digestive system is either absent or incomplete with a single opening- the mouth, anus is absent. Circulatory, respiratory and skeletal system are absent. Excretion and osmoregulation is brought about by protonephridia or flame cells. Ammonia is the chief excretory waste product. Nervous system is of the primitive type having a pair of cerebral ganglia and longitudinal nerves connected by transverse commissures. Sense organs are poorly developed, present only in the free living forms. Basically hermaphrodite with a complex reproductive system. Development is either direct or indirect with one or more larval stages. Flatworms have a remarkable power of regeneration. The phylum includes about 13,000 species. Here Dugesia and Fasciola hepatica will be described as the type study to understand the phylum. Some of the medically important parasitic helminthes will also be discussed. Evolution of parasitism and parasitic adaptations is of utmost importance for the endoparasitic platyhelminthes and will also be discussed here. Dugesia (Euplanaria) HABIT AND HABITAT: Dugesia is a free living inhabitant of cool and clear water of freshwater ponds, lakes, streams and shallow water rivers. They are gregarious i.e. they live in groups attached to the undersurface of leaves, logs, rocks and other debris during day-time. They become active in dark. Dugesia are worldwide in distribution. MORPHOLOGY: Body of Dugesia is thin, flattened, leaf-like and oval with a definite polarity. A full grown Dugesia measures about 50mm in length and is greyish, brownish, or blackish in color. The dorsal surface is darker in color than the ventral surface. Ventral surface is covered with cilia that helps in locomotion. However, a narrow strip all along the margin of the ventral surface is non-ciliated and is known as the adhesive zone. It helps in adhesion. The anterior end of the body is differentiated into a broad, blunt and triangular head that bears two lateral projections called the auricles. Present on the mid-dorsal line of the head are two black eye spots (Fig. 1). A small neck like constriction separates the head from the main body. Mouth is present in the middle of the body, on the mid-ventral surface. In sexually mature worms the genital aperture is present on the ventral surface a little behind the mouth. Numerous microscopic excretory apertures are situated on the dorsal surface (Fig.1).

BODY-WALL: The body-wall is made up of an outer epidermis and inner muscle layer. The two layers are separated by a basement membrane. The space between the muscle layer and gut is filled up with parenchymal cells (Fig. 2).

EPIDERMIS: The epidermis is made up of a single layer of large cuboidal epithelial cells. It is ciliated on the ventral surface. Interspersed in between the epidermal cells are sensory cells, adhesive glands and mucus gland cells. The mucus gland cells provide a mucus coating, forming a slime track on which the animal crawls. They are more abundant on the ventral surface in the anterior part of the body. Adhesive gland cells secrete a sticky substance that

helps in the attachment of the body to the substratum, cementing of eggs and capturing the prey. Both mucus and adhesive gland cells are seated deep in the mesenchyme and have long narrow ducts which pass through the muscle layer, and the basement membrane and finally open on the surface of the epidermis. Present in the epidermal cell, mostly on the side, are many rod shaped hyaline bodies known as rhabdites. Rhabdites are secreted by rhabdite gland cells present below the epidermis (Fig. 2). The exact function of rhabdites is not known but it is believed that they help in capturing the prey, in locomotion and give protection to the body. When the rhabdites are discharged they come in contact with water and swell and form a thick, opaque adhesive layer around the body which gives protection to the animal. BASEMENT-MEMBRANE: Just beneath the epidermis is a thin structureless basement- membrane. The basement membrane not only provides surface for the attachment of epidermal cells, but it also acts as a partition between the epidermis and the muscle-layer. The basement- membrane bears pigments and helps in maintaining the general form of the body. Basement membrane serves as the elastic membrane. MUSCLE –LAYER: Beneath the basement-membrane is the muscle layer. It consists of outer circular muscle fibres and inner longitudinal muscles. Also present are oblique or diagonal fibres arranged in a vertical manner. The longitudinal muscles on the ventral surface are more strongly developed than on the dorsal surface. Dorsoventral muscles are present between the dorsal and ventral surface. PARENCHYMA OR MESENCHYME: Lying between the muscular layer and the alimentary canal is the parenchymatous tissue which are loose connective-tissue cells that act as a packing material. Its fluid filled spaces provide turgidity to maintain the body form. It contains free wandering amoeboid cells that remain in the formative state. These formative cells bring about regeneration of damaged or lost parts. Mesenchymal cells also take up the circulatory function by conducting food and other metabolic products from one part of the body to another. LOCOMOTION: Although Dugesia lives in water it does not swim but moves about by gliding. While gliding the head is slightly raised. The cilia present on the ventral surface move in the backward direction and help the organism to move forward over a slime track. The slime track is secreted by the mucus gland cells present in the epidermis. The mucus affords a grip to the cilia and also protects them from injury by the substratum. Sometimes the animal also crawls. Crawling is brought about by the movement of the muscles. Elongation of the body is brought about by the contraction of the circular and oblique muscles. The anterior end of the body then gets firmly fixed onto the substratum by mucus. The longitudinal muscles then contract and pull the animal forward. The longitudinal muscles contract alternately on the right and left side of the body. Thus, the head also bends alternately on the right and left as the animal moves forward in a wavy manner. Dugesia can change its direction with the help of the oblique muscles. DIGESTIVE SYSTEM: Dugesia has an incomplete digestive tract with a single opening – the mouth. Anus is absent in them. Alimentary canal consists of mouth, pharynx and intestine (Fig. 3). MOUTH: Mouth is a small oval aperture situated on the mid ventral surface of the body. In the absence of the anus, the mouth serves the function of both ingestion and egestion. PHARYNX: Mouth opens into a cylindrical pharynx through a small buccal cavity. The thick walled muscular pharynx lies in a pharyngeal cavity or pouch bounded by a muscular sheath called the pharyngeal sheath. The pharynx can be everted out through the mouth and helps in

feeding. When in retracted condition the pharynx remains enclosed in the muscular sheath.

INTESTINE: The pharynx leads into the intestine which divides into three branches. One of these branches run forward along the middle line upto the head and the other two run backwards upto the posterior end. All the three branches give off numerous lateral ramifying branches. Thus, the intestine with its ramifying branches form a network and occupy a major

part of the body. All the branches end blindly as there is no anal aperture. The much branched intestine increases the surface area for the digestion, absorption and distribution of food. Columnar epithelial cells line the inner walls of the intestine. FEEDING AND DIGESTION: FOOD: Dugesia is carnivorous. It feeds on dead or living organisms, mostly the crustaceans, worms, insect larvae and snails. It can also subsist on body fragments of larger animals, living or dead. INGESTION: Dugesia can detect its prey from some distance with the help of chemoreceptors present on the sides of the head. On detecting the food, it moves towards it, creeps over it with the head slightly raised and entangles the prey in slimy secretions of mucus glands and rhabdites. It now holds the anterior end of its body over the prey and immobilizes it. Pharynx is then everted through the mouth which encloses the food. The food is then ingested by the peristaltic action of the pharyngeal wall. Smaller prey is ingested as such, while the larger ones are first broken down into smaller particles by the pumping action of the pharynx aided by the digestive juices secreted by the pharyngeal glands and then ingested. DIGESTION: Digestion is both extracellular and intracellular. In the pharynx, the food is broken down by the pumping action of the pharynx and is then acted upon by extruded digestive juices. The digestive juices include pharyngeal enzymes and endopeptidase enzymes of gland cells of the intestine. This is extracellular digestion. Partially digested and liquefied food is then pumped into the intestine by peristaltic action. Intracellular digestion takes place in the phagocytic cells lining the intestine. Digested food diffuses through the walls of the intestine into the mesenchyme. Mesenchyme helps to distribute the digested food to all parts of the body in the absence of the circulatory system. Reserved food is stored in the form of fat and sometimes as protein globules in the epithelial cells of the intestine. Undigested food is egested through the mouth as there is no anus. Dugesia can live without food for long periods. During this period they obtain their nourishment by dissolving their reproductive organs, parenchyma and muscles. The missing body parts are regenerated when they start feeding again. RESPIRATION: Special respiratory organs are lacking. Exchange of respiratory gases takes place through the general body surface by diffusion. EXCRETORY SYSTEM: Excretion is brought about by specialized cells called flame cells or protonephridia. The excretory system consists of two networks of longitudinal canals running throughout the length of the body, one on each side. These open to the outside by several minute pores called nephridiopores present on the dorsal surface. Each pair of trunk is coiled and is connected to one another by transverse vessels in the head region. Each longitudinal net work gives off many branches which in turn divide into extremely fine capillaries (Fig. 4A). Many of these capillaries bear flame cells or flame bulb at its tips. Flame cells are modified mesenchymal cells. Each flame cell is large and gives out numerous branched protoplasmic processes (resembling pseudopodia) in the surrounding mesenchyme. In the centre of the cell is a conspicuous bulbous cavity or cell lumen containing a bunch of cilia whose beating motion gives the cell a flickering appearance like a candle flame and hence the excretory cells are known as flame cells (Fig. 4B). Each cilia arises from a basal granule. The cytoplasm is appressed to one side and contains a rounded nucleus, some excretory globules and vacuoles.

PHYSIOLOGY OF EXCRETION: The excretory waste material enters the cavities of the flame cells from the mesenchyme by the simple process of diffusion. The continuous beating of the cilia causes hydrostatic pressure which keeps the fluid waste circulating through the longitudinal trunk. The walls of the lateral excretory canals are also ciliated at least in parts which further keeps the fluid moving through

them. As the fluid waste passes through the longitudinal excretory canals, useful substances are reabsorbed by the walls of the canals, while the excretory waste material along with excess of water is expelled out through the nephridiopores. A considerable amount of waste is also expelled out through the mouth and general body surface. NERVOUS SYSTEM: Dugesia has a primitive type of centralized nervous system. It consists of a pair of cerebral ganglia at the anterior end, a little behind the eyes. The two cerebral ganglia join to form a bilobed brain. The brain is made up of connecting transverse fibres and nerve cells. Numerous nerves extend forward and laterally from the brain to the head, eyes and auricles. Posteriorly, the brain gives out two pairs of longitudinal nerve cords which run backwards giving rise to numerous transverse branches to both the external and internal parts of the body. These lateral transverse branches anastomose medially with branches of opposite sides to form transverse commissures (Fig. 5A). This gives them a ladder- like appearance and hence the nervous system is sometimes called as the ladder type of nervous system. In addition to the centralized nervous system, Planaria also possesses a sub-epidermal nerve- plexus like cnidarians. Brain receives the stimuli from the sense organs and conveys them to different parts of the body. SENSE ORGANS: Sense organs consists of a pair of eyes, some scattered tangoreceptors, auricular organs, and rheoreceptors. EYES OR OCELLI: A pair of eyes are present as rounded dark spots near the anterior end on the dorsal surface. It consists of a cup shaped pigment screen. Inside the cup are many sensory cells that act as light receptors (Fig.5B). Spatial arrangement of pigment cells and light sensitive cells render the animal capable of crude discrimination of direction of light. The pigment cup serves as a shield and light can enter only through its opening to stimulate the photosensitive expanded ends of sensory cells. The light stimulates the sensory cells which transmit the impulse to the brain by optic nerve. The eye of Planaria lacks a lens and refractive apparatus. No image formation takes place in them. They can only perceive the difference between light and dark. If the eyes are removed from Dugesia they can still react to light but slowly and less accurately. This is because it possesses some light sensitive cells over the general body surface.

AURICULAR ORGANS: A few sensory cells are found sunken in grooves or pits in the head region and are called auricular organs. They are well provided with nerves, are devoid of gland cells and rhabdites. These are organs for chemical sense. They help in detecting food and directing the animal towards it. TANGORECEPTORS: These are sensory cells, tactile in nature distributed abundantly on ventral surface all over the body especially around the mouth, lateral margins and at the anterior end. The sensory bristles of these cells project over the epidermis beyond the level of cilia and give the sensation of touch.

RHEORECEPTORS: Sensory cells sensitive to water currents. Dugesia is positively rheotactic i.e. it travels against the water current with its head forward. REPRODUCTIVE SYSTEM: Dugesia reproduces both asexually and sexually. The reproductive organs develop temporarily only during breeding season (during summer). ASEXUAL REPRODUCTION: Asexual reproduction takes place by transverse binary fission. Fission occurs when the animal has attained maximum size. The posterior end adheres firmly while the anterior end moves forward so that a constriction appears behind the pharynx (Fig. 6). The constriction gradually deepens, finally dividing the animal into two halves each of which regenerates its missing parts.

SEXUAL REPRODUCTION: Dugesia is hermaphrodite. Although hermaphrodite, cross fertilization is a rule because of protandry ( male reproductive organs developing before the female).

MALE REPRODUCTIVE ORGANS: Male reproductive system consists of testis, vasa efferentia, a pair of vas deferens and penis. Numerous small and rounded testis are present on the right and left borders of the body. Each testis opens into a small vasa efferentia which in turn opens into a vas deferens of its side. The right and the left vas deferens unite at the middle of the body to form a median duct which opens into a muscular penis. The median duct is swollen at the base of the penis to form a seminal vesicle where the sperms are temporarily stored. The penis is thick walled, muscular, copulatory organ consisting of two parts: A proximal part called penis bulb surrounded by unicellular prostate glands and the distal part called penis papilla which is capable of protruding through the gonopore (Fig. 8). The secretions of the prostrate gland serves to stimulate the sperms into activity.

FEMALE REPRODUCTIVE ORGANS: It consists of ovaries, oviduct, vitelline glands and bursa copulatrix. A pair of small rounded ovaries are situated at the anterior end of the body one behind each eye. Each ovary opens into an oviduct. The proximal part of oviduct stores sperms which are received during cross fertilization and is called vesicula seminis or seminal receptacle or bursa copulatrix. The two

oviducts meet at the posterior and ventral part of the body to form a common oviduct which opens into the genital atrium. Numerous small branching vitelline or yolk glands surround the oviduct and open into it by fine vitelline ducts. The oviduct is thus actually an ovo-vitelline duct. Yolk cells produced from the yolk glands pass into the oviduct. Numerous small cement glands open into the common oviduct and genital atrium. The bursa copulatrix also opens into the atrium. The genital atrium opens to the outside by a genital aperture situated on the ventral side behind the mouth (Fig. 8). COPULATION, FERTILIZATION AND DEVELOPMENT: During copulation the two worms come together by their ventral surface facing in the same direction (Fig. 9). The penis papilla of each worm dilates, protrudes through the gonopore into the copulatory sac of the other. Copulation here means mutual exchange of sperms. Sperms from the seminal vesicle of one worm are passed into the copulatory sac of the other where they stay only for a short while. The Dugesia then separates and the sperms migrate to the oviduct and reach seminal receptacle where they fertilize the ova as they descend from ovary. The fertilization is thus internal. The fertilized egg passes down the oviduct and gets surrounded by yolk cells secreted by yolk glands. Thus, a capsule or cocoon is secreted around the zygote as it passes into the genital atrium. The cocoon is provided with an adhesive stalk secreted by the cement glands. The egg capsule comes out of the body of the worm and gets attached to stones, aquatic weeds etc. by their adhesive stalk. Development is direct without any larval forms. Each zygote in the capsule develops into a young worm in about two weeks. Yolk cells provide nourishment for the developing young ones within the cocoon. On maturation the capsule wall ruptures and a young worm emerges out. The young one resembles the adult in all respects except that it is smaller in size and is not sexually mature. They feed, grow and develop into a mature adult. Reproductive organs degenerate and disappear after each mating and the worm reproduces asexually by fission. They again develop during the next breeding season. During the breeding season, each worm may copulate many times and lay a succession of capsules at intervals of few days. FREGENERATION: Dugesia has a tremendous power of regeneration. Regeneration involves two processes: 1. EPIMORPHOSIS: In this process, missing parts are formed. 2. MORPHOLLAXIS: It involves the adjustment and coordination between the old tissue and regenerated tissues. The process of regeneration occurs in the following manner: If the animal body is cut into two or more pieces, each one will form the lost parts. A piece from the middle will always regenerate a head towards its anterior end and tail towards its posterior end. Each piece maintains its polarity. The metabolic activity is highest in head and gradually decreases towards the tail end. Correspondingly, anterior end of each piece, having greater metabolic activity regenerates the anterior part of the body and posterior end piece having lesser metabolic activity regenerates the posterior part of the body (Fig. 7). This capacity of regeneration from anterior to posterior end of the body due to a gradual difference in the intensity of metabolic activity is called as the axial or metabolic gradient. Regeneration is brought about by the free formative cells of the parenchyma. These cells migrate to the cut surface and by repeated division produce new tissues. However, the regeneration is not always perfect because at times the two halves fail to regenerate the sex organs and each grows into an asexual individual. Also, if the anterior end of the animal is cut longitudinally into two or more

parts, then each part would develop a new head, resulting in a heteromorph or many headed monster. Fasciola hepatica L.,Fasciola means a small bandage; Gr., Hepar means liver Commonly known as sheep liver fluke. It is also known as Distomum hepatica. It was the first trematode to have been discovered by de Brie in 1379. Its life cycle was first worked out by Thomas in 1883. It is the largest and commonest liver fluke that lives as an endoparasite in the bile duct of sheep. It causes the economically important disease liver rot in sheep. MORPHOLOGY: Soft, oval, dorsoventrally flattened leaf like body about 15-30mm long and 10-15mm broad, green or brown in color. Anterior end of the body has a conical projection known as the oral cone or cephalic cone or the head lobe. The body widens in the middle and slowly tapers towards the posterior end. There are two suckers: An anterior sucker or oral sucker situated at the tip of the oral cone and a posterior sucker or acetabulum situated midventrally near the broad portion of the body. Both the suckers are organ of attachment. They attach to the body of the host by vaccum. The oral sucker along with the mouth also helps in the ingestion of the food. Situated in the middle of the oral sucker is the mouth from the margin of which radiate out muscles to the periphery of the oral sucker. A little in front of the acetabulum on the midventral side is present a small genital aperture or gonopore. During breeding season, a temporary opening known as opening of the laurers canal appears on the dorsal surface, in the middle 1/3rd region of the body. A single excretory pore is situated ventrally at the posterior end of the body (Fig. 10). The body surface is marked by the presence of a number of conical projections- the spinules which are cuticular extensions of the body. BODY WALL: Body wall consists of three layers: Tegument, musculature and mesenchyme (Fig.11). TEGUMENT: Body is bounded by an outer layer of non-ciliated, syncytial and anuclear (non- nucleated) tegument. It bears numerous microscopic backwardly directed spinules or spines or scales. The spines help the animal to anchor to the bile passage of the host and provide protection to the body. Its matrix contains mitochondria, endoplasmic reticulum, ribosomes, golgi complex and various secretory bodies, vacuoles and pinocytic vesicle suggesting that it is a highly synthesizing layer and metabolically active which is of physiological importance to the parasite. Microvilli have also been identified on the outer surface of the tegument. The microvilli increase the surface area for the absorption of food. Tegument is connected by narrow cytoplasmic tubes to the cytoplasmic processes of certain tegumental secreting cells lying in the parenchyma. FUNCTIONS OF TEGUMENT: 1. Absorption of food from the host body. 2. Synthesis and secretion of materials. 3. Respiratory and sensory function. 4. Excretion and osmoregulation. 5. Outer layer is made up of mucopolysaccharide and phenol which makes the tegument resistant to the host enzymes thereby protecting the worm from being digested by the host.

BASEMENT MEMBRANE: Below the tegument lies a thin, delicate basement membrane. It separates the tegument from the muscle layer. MUSCLE LAYER: It consists of an outer layer of circular muscle fibres, middle longitudinal muscle fibres and inner oblique or diagonal fibres. All the muscle fibres are of smooth type. The muscles form stout bundles of radial fibres in the suckers. MESENCHYME OR PARENCHYMA: Numerous unicellular gland cells are found sunken in the parenchyma which open out on the surface of the tegument by protoplasmic projections (Fig.11). They form the packing material between the muscle layer and the internal organs. Parenchyma performs the function of skeleton and transporting system.

DIGESTIVE SYSTEM: Fasciola has an incomplete digestive tract with a single opening the mouth. Anus is absent. Mouth is situated ventrally at the apex of the head lobe, surrounded by oral sucker. The mouth leads into a muscular pharynx, having a narrow lumen and thick walls and provided with pharyngeal glands. Pharynx leads into a short and narrow oesophagus which in turn opens into

the intestine (Fig. 12). Intestine immediately divides into two branches which run on either side of the body and terminates blindly at the posterior end. The two branches give off numerous irregular side branches known as caecae or diverticulum all along its length filling up a greater part of each side of the body (Fig. 12). The caeca of the inner side are short and simple while those of the outer side are large and branched. Anus is absent. FEEDING AND DIGESTION: With the help of the oral sucker and pharynx, Fasciola sucks up the hosts bile and blood from the walls of the bile duct. Glucose, Fructose and Amino acids directly diffuse into the body of the fluke through the tegument. Digestion is extracellular taking place in the intestinal caecae. Digested food diffuses out into the surrounding parenchyma and then distributed to different parts of the body. The highly branched intestine helps in the distribution of food to all parts of the worm. Undigested food is given out through the mouth. Food is stored in the mesenchyme and muscles as glycogen and fats.

RESPIRATORY SYSTEM: Respiratory organs are lacking. As the animal is an endoparasite, respiration in them is anaerobic. Glycogen stored in the body as reserve food undergoes anaerobic glycolysis to form pyruvic acid which is further decarboxylated to form acetyl group and carbondioxide. Acetyl group combines with CoA to form Acetyl CoA. Acetyl CoA then condenses to form volatile

fatty acids- Acetic, Lactic and Propionic acids. Carbondioxide diffuses out through the general body surface while the fatty acids are excreted out through the excretory system. EXCRETORY SYSTEM: The excretory system consists of protonephridia or flame cells scattered throughout the parenchyma. Flame cells are connected to one another by a network of excretory ducts (Fig. 13A). Each flame cell is a modified mesenchymal cell and has structure similar to one described in Dugesia (Fig. 13B). All the flame cells open by an excretory tubule into larger excretory tubes. The excretory tubes of the anterior part of the body open into four ducts: two dorsal and two ventral which in turn unite posteriorly to form a common median longitudinal excretory duct. This median ductl runs up to the posterior ventral end of the body and opens to the outside by a common pore called the nephridiopore. Excretory vessels of the posterior part of the body open directly into the median longitudinal excretory duct. Excretory products like ammonia, fatty acids and other waste metabolites diffuse from the surrounding parenchyma into the flame cell. The excretory fluid is further kept moving through the tubules by the ciliary movement and finally is excreted out through the excretory pore.

EXCRETORY WASTE MESENCHYME FLAME CELL

EXCRETORY PORE MEDIAN EXCRETORY CANAL VESSELS

NERVOUS SYSTEM: It consists of a pair of cerebral ganglia situated one on either side of the oesophagus. Nerves are given out to the head lobe and to the hind part of the body from the cerebral ganglia. A nerve collar exists around the oesophagus and connects the cerebral ganglia. Three pairs of longitudinal nerves, dorsal, ventral and lateral are given out posteriorly from these ganglia. Of these longitudinal nerves, the lateral nerve cords are well developed and extend upto the posterior end and on their way are connected by transverse commissures. As they run throughout the length of the body they give out many small branches, some of which form plexus (Fig. 14). Sense organs are lacking in them.

REPRODUCTIVE SYSTEM: Highly branched and complete genital organs lie between the two branches of the intestine. Fasciola hepatica is a hermaphrodite.

MALE REPRODUCTIVE SYSTEM: A pair of testes is present, with one testis lying behind the other in the posterior half of the body. From each testis arises a small duct called vas deferens which extends anteriorly towards the ventral sucker and meet with its counter part to form a common duct. This dilates into a muscular, sac-like seminal vesicle before opening into an ejaculatory duct. Seminal vesicle stores the sperms. The ejaculatory duct is surrounded by numerous unicellular prostrate glands. The alkaline secretions of the prostrate glands help in the free movement of sperms during copulation. The ejaculatory duct in turn opens into the genital atrium. The end of the gonoduct is modified into a male intromittent organ known as cirrus or penis (Fig. 15). Cirrus, prostrate glands and seminal vesicle are enclosed in a pouch known as cirrus sac. The genital atrium opens outside by a common gonopore situated mid-ventrally in the anterior half of the worm.

FEMALE REPRODUCTIVE SYSTEM: A single, highly branched ovary lies in front of the testis. It occupies anterior one-third of the body. All the branches of the ovary open into a short and narrow tube called the oviduct. The oviduct extends backwards to meet the median vitelline duct. From the junction of the oviduct and median vitelline duct arises the uterus. Uterus is a long, wide and highly convoluted tube

that runs anteriorly upto the genital atrium near the base of the cirrus. Uterus contains a large number of fertilized shelled eggs or capsules. Surrounding the junction of oviduct, median vitelline duct and uterus are a cluster of unicellular glands called Mehlis gland. Terminal part of the uterus has muscular walls and is called the metraterm. Metraterm ejects the eggs and also sometimes receives the cirrus during copulation. Genital atrium opens to the outside by a pore known as gonopore situated mid-ventrally close to the ventral sucker (Fig. 15). During the breeding season, a temporary opening arises from the oviduct on the dorsal surface known as Laurers canal which acts as the vagina and receives the penis during copulation. VITELLINE GLANDS: Associated with the female reproductive system are the vitelline glands. These glands occur as cluster of follicles, and are scattered throughout the length on the lateral side of the body. The follicles of each side are connected by minute interconnected ductules to a large longitudinal vitelline duct. The longitudinal vitelline ducts of both sides are connected by a transverse vitelline duct almost in the middle of the body. The transverse duct swells up in the centre to form the yolk reservoir. A small median vitelline duct joins the yolk reservoir to the oviduct (Fig. 15). The vitelline glands produce yolk cells or vitelline cells. These cells contain abundant yolk for nourishing the embryo. They also contain granules called the shell globules or vitelline globules. As the egg mass moves through the oviduct, a group of vitelline cells surround it. Shell globules are extruded out from the vitelline cells which coalesce to form a thin membrane that becomes the outer covering of the egg shell. More globules are released and the shell is built up from within. Egg shell is transparent and is made up of protein. FUNCTIONS OF MEHLIS GLAND: 1. Lubrication of uterus for the safe passage of the eggs. 2. Activation of spermatozoa by their secretion. 3. Gland also secretes a phospholipid that may effect the release of the egg shell precursor from vitelline glands. 4. It also produces free phenols from mucopolysaccharides and activates phenolases thereby allowing the oxidation of phenols to quinone, thus helping in the tanning of egg shell. 5. It provides a membrane which serves as a template on which shell droplets accumulate to form egg shell.

COPULATION AND FERTILIZATION: Although hermaphrodite, cross fertilization is a rule because of protandry. Sometimes self fertilization may also occur. Copulation occurs in the bile duct of sheep. The cirrus of one worm is inserted into the laurers canal of the other worm and sperms are ejaculated. Sperms travel up to the distal end of the oviduct where fertilization takes place. The eggs once released from the ovary get fertilized in the oviduct. Yolk is provided by the yolk cells which also carry the material for egg shell formation. Since the yolk occurs outside the egg shell the fluke eggs are ectolecithal. LIFE-CYCLE: It is digenetic i.e. requires two host to complete its life cycle. DEFINITIVE HOST: Herbivores especially sheep. Man is only an incidental host. INTERMEDIATE HOST: Fresh water snail- Lymnaea truncata.

RESERVOIR: Sheep. HABITAT: It lives in the biliary passage of liver, gall bladder or associated ducts of sheep and other herbivores. EGGS: The number of eggs produced are enormous,5,00,000 in its life time. The high rate of egg production is a parasitic adaptation for its endoparasitic mode of life. Eggs are large 140x75µm, ovoid and brownish yellow. The eggs when voided out of the host body are operculated and unembryonated i.e. they contain a large unsegmented ovum surrounded by yolk cells which provide yolk and shell globules. Each egg is enclosed in a proteinaceous shell or capsule. CLEAVAGE AND EARLY DEVELOPMENT: Cleavage is holoblastic and unequal. The first division of the zygote results in a small propagatory cell and a large somatic cell. The somatic cell divides repeatedly to form the ectoderm of the larva. The division of propagatory cell results in the formation of two daughter cells: a propagatory cell like itself and a somatic cell. The somatic cells, by further division form the endoderm and mesoderm of the embryo while the propagatory cell pass into the posterior end of the embryo and divide repeatedly to give rise to a number of germ cells. Further development is not possible in the uterus of the liver fluke. A large number of encapsulated embryos are given out from the gonopore of the fluke into the gall bladder of the host, from where they enter the duodenum of the host. The encapsulated embryos are finally voided out along with the faeces of the host.

Eggs pass from the uterus of the worm into the gall bladder of the host Bile or hepatic duct

Expelled with faeces Duodenum

EMBRYONATION TAKES PLACE IN SOIL OR WATER: FACTORS RESPONSIBLE FOR EMBRYONATION TO TAKE PLACE: 1. Moisture—60% 2. Temperature---22-250c 3. pH --6.5 It normally requires 14-17 days for embryonation to take place but the process may be effected by other environmental factors.

FACTORS RESPONSIBLE FOR HATCHING TO TAKE PLACE: 1. TEMPERATURE 2. OSMOTIC PRESSURE 3. LIGHT: It has been suggested that on exposure to light, a hatching substance is produced by the miracidium larva within the egg shell which acts on the operculum binding material from inside and the operculum flies open and the miracidium larva escapes out. The

hatching substance is probably a proteolytic enzyme. An alternate hypothesis putforth by Wilson suggests that light stimulates the miracidium into activity which in turn results in altering the permeability of the internal surface of the viscous cushion. Change in permeability allows the egg contents to reach the cushion which then becomes hydrated thereby increasing the internal pressure and finally rupturing the operculum and liberating the miracidium. In two weeks time a small ciliated larva known as miracidium larva emerges out of the egg shell by forcing the operculum. MIRACIDIUM LARVA: Miracidium means a little boy. It is a free swimming larva. Body is more or less fusiform and ciliated. The body is covered by epidermal plates. Number of plates is characteristic of a species. Its anterior end is broader than the posterior end and is produced into an apical lobe or apical papilla. X-shaped eye spot are present at the anterior end behind the apical lobe. At the anterior end is a median apical gland which is believed to release a proteolytic enzyme that aides in the process of penetration. One to several bilaterally arranged pair of penetration glands secrete a mucoid substance which assists in attachment to the snails tissue. A pair of protonephridia are present, each opening to the outside by a nephridiopore. Scattered in the posterior part of the body are germinal cells which are a mass of undifferentiated cellular material (Fig. 16). Miracidium does not feed. Miracidium freed from the egg shell can live only for a few hours and during this time it swims about randomly in search of a suitable host. If it fails to encounter a suitable host within 24 hours, it dies. Miracidium demonstrates a high degree of selectivity for their host. Physical and chemical factors present in the hosts gut serve as the determinants of compatibility or incompatibility of the host and the miracidium. FACTORS AFFECTING THEIR SELECTIVITY: 1. Chemical attractants (Chemotactic factors). 2. Mucus: Miracidium are highly selective and can perceive the mucus secreted by a particular species of snail. However, many miracidium reach their intermediate host purely by chance, although tropism (phototropism or geotropism), temperature, salinity, pH, may guide them to areas of host concentration and thereafter chemotaxis would operate. Age of mollusk is probably also a controlling factor in selection and entry of miracidium.

ENTRY OF MIRACIDIUM INTO THE BODY OF THE INTERMEDIATE HOST: The miracidium penetrates through the body surface of the snail in the head-foot complex or the tentacles. The miracidium probes with their anterior end and attempt to bore into almost any object, including various species of unsuitable snails and other animals like planarians or even unanimate objects. These larvae may keep trying to enter an unsuitable host or an unsuitable part of the hosts body like the shell until they die of exhaustion. Once the miracidium comes in contact with the head-foot complex of the suitable snail, Lymnaea truncata, it attaches itself to the body of the snail by the apical papillae and perform boring action. Penetration glands produce a mucoid substance that enables the miracidium to adhere to the snail. The substance also functions in lubrication. A cytolytic substance is also produced by the apical glands which break down the host tissue and make a perforation in the skin. Apical papillae elongates and works its way through the snails epithelium like a drill. Miracidium sheds its cilia and thrusts itself into the soft tissues of the snail and ultimately makes its way into the digestive gland of the snail.

SPOROCYST: Sporos means seed and kystis means cell or bladder. The miracidium within the snail elongates to become a motile, vermiform, sac like larva called sporocyst. Apical glands, penetration glands, apical papillae, eye spot present in the miracidium larva all degenerate and disappear. The sporocyst retains all the body wall layers of the miracidium except the ciliated epidermis which is lost in the process of penetration and is soon replaced by a thin cuticle. Protonephridia of each side soon divide and the two so formed open through a common excretory duct. Sporocysts are essentially germinal sacs containing germinal cells which multiply and form new germinal masses (Fig. 17). Sporocyst moves about in the hosts body by muscular contractions, absorbing nutrition from it. Within the sporocyst, next larval stage is developed from the germinal sac through a process which is a repeat of formation of miracidium from egg. These are called rediae. REDIA: Named after the Italian scientist Francesco Redi Rediae emerge from the sporocyst by rupture of its bodywall. Each redia is elongate and cylindrical. Bodywall consists of the usual layers viz. cuticle, musculature and the mesenchyme. Rediae show the beginning of the adult characteristics each having developed an oval sucker and embryonic gut. It bears a mouth at its anterior end surrounded by sucker. Mouth leads into a short muscular pharynx followed by an elongated sac like intestine. Numerous unicellular pharyngeal glands open into the pharynx. Redia feeds on the hosts digestive juices. Near the anterior end of the body is a ring like muscular swelling called the collar. A birth pore is present at the anterior end, a little behind the collar through which the next generation larvae escape out. The protonephridia branches further and forms a much elaborate system. All the flame cells of each side open out through a common excretory duct. Present ventrolaterally at the posterior end are the Lappets or Procruscula which are the conspicuous feature of rediae (Fig. 18). The body of the larva is packed with germ balls and germ cells. The rediae moves about in the hosts tissue. The movement is brought about by the muscular contractions of the body. Collar and lappets aid in the movement. Moving rediae enters into various organs of snail but prefer to migrate to the digestive gland. The next larvae i.e. the cercariae develop within the rediae. These emerge out through the birth pore. During the summer months when sufficient nourishment is available, instead of cercariae a second generation of rediae is formed. However, during the winter months cercariae are formed. Each redia forms 14-20 cercariae.

CERCARIA: Kerkos means a tail. The cercaria larva escapes out of the rediae through the birth pore and enters the digestive gland of the snail. Cercaria larva is a tailed immature sexual form that resemble the adults in general body form. Mature cercariae possesses two suckers (an oral sucker and ventral sucker), a bifid gut and a tail. The tail is a secondary adaptation. The usual body layers are present (Fig. 19).

Excretion is by flame cells. The basic pattern of flame cells is same as that of the adults. Cercariae may have eye spots or photoreceptors consisting of sensory and pigment containing cells (Fig. 19). Numerous cystogenous glands are present below the body wall, the secretions of which help to form the cyst wall to become the next larva-The metacercariae. Germ cells represent the rudiments of adult genital system. Cercariae emerge from the mollusk into the water and alternate the periods of swimming by lashing the tail violently with short periods of resting. Life span of cercariae is limited. A single miracidium can produce a large number of cercariae. Life cycle in snail is completed from 60 days to 90 days i.e. from miracidial penetration into snail to the emergence of cercaria from the snail. Released cercariae show negative geotropism, positive phototropism, and positive thermotropism. FACTORS THAT STIMULATE CERCARIAE EMERGENCE: 1. Temperature: Between 9-260C favours its emergence. Above 260C the snails cannot survive. 2. Rainfall has a stimulating effect. 3. Light has a positive effect. 4. Circadial rhythm: Maximum production of cercariae is between midnight and 1a.m. On emergence the cercariae attach to objects like vegetation, grass, blades of some aquatic weeds etc. in water and shed off its tail. The cystogenous glands secrete a protective cyst wall about themselves and soon disappear. Encysted cercariae are round in shape and are called as metacercariae or Adolescercaria or juvenile fluke. Metacercariae are prototype of the adult (Fig.20). Metacercariae is the infective stage. When swallowed by the final host, cyst wall is dissolved by the digestive enzymes of the host, and young fluke is released. Uncysted cercariae if swallowed by the primary host are destroyed by the acidic juices of the stomach. Cyst wall is resistant to the acidic juices of the stomach. Further development of the metacercaria takes place only if swallowed by the final host, the sheep. However, the metacercariae are not infective until 12 hours after encystment. The sheep gets the infection while grazing on the aquatic weeds. Once the metacercaria enters the lumen of the intestine, the cyst wall is digested by the action of the hosts digestive juices and the young fluke emerges which bores its way through the wall of the intestine and enters the liver through the hepatic portal system. The young flukes remain in the liver for 7-8 weeks feeding mainly on blood. They then enter the bile duct and bile passages where they become sexually mature. The incubation period in sheep is 3-4 months. Adult fluke lives in the sheep for about 5 years. In man it lives relatively longer, for about 9-13 years.

ADULT LIVER FLUKE EGGS (In the bile duct and biliary passage of sheep)

ENTERS THE DEFINITIVE HOST VOIDED OUT ALONG WITH THE FAECES OF THE HOST

SHEEP

METACERCARIA OUTSIDE MIRACIDIUM LARVA (Encysted Larva----Infective Stage) (Infective stage for intermediate host)

SNAIL

CERCARIA LARVA REDIA LARVA SPOROCYST LARVA

MODE OF INFECTION: Oral PORT OF ENTRY: Alimentary canal INFECTING AGENT: Metacercariae SITE OF LOCALIZATION: Bile duct of herbivores PATHOGENICITY: Causes fascioliasis. In traversing the liver tissue, it causes parenchymal injury. EFFECT ON HERBIVORES: 1. The adult causes biliary obstructive symptoms. In bile duct, it causes inflammation and hepatitis, intermittent obstruction and dilation of the biliary tract with considerable thickening of its wall, followed by calcification and formation of gall stones. The parasite may cause fibrosis of bile duct and gall bladder. 2. Heavy infestation may upset the normal metabolism of liver. This is due to haemorrhage caused and irritation inflicted by cuticular spines. This disease is called liver rot. Symptoms of liver rot are more acute in lambs than in sheep. Frequently death may result due to cerebral apoplexy or acute anaemia. 3. Apetite declines, rumination(chewing of cud) becomes irregular and at times there is fever and an increase in the respiratory activity. 4. Conjuctiva becomes whitish yellow. 5. In sheep the wool becomes dry and brittle and falls off. 6. Lactation and breeding are reduced. 7. In general, growth, milk yield, body weight and wool of sheep may be reduced.

EFFECT IN MAN: 1. In man it causes more severe inflammatory response and hepatitis resulting in the massive destruction of the liver tissues and inflammation of the bile duct. 2. Some larvae penetrate the liver and diaphragm to reach the lung, brain or other tissues. 3. Initially, the patients suffer from fever, eosinophilia and hepatomegaly. Later they develop acute epigastric pain, anaemia and obstructive jaundice. Occasionally, ingestion of raw liver of infected sheep results in a condition known as halzoun meaning suffocation. Halzoun is more common in Lebanon, and other parts of Middle East and North Africa.

DIAGNOSIS: 1. Eggs in feces. 2. Duodenal and bile intubation.

PROPHYLAXIS: 1. Erradication of the intermediate host, Lymnaea truncata from the water courses inhabited by 2. sheep. Ducks feed on snails and can be used as biological control. 3. Breeding of snails can be checked by removing vegetation from ponds and streams that they inhabit. 4. Snail population can be checked by adding copper sulphate solution in ponds and ditches or by 5. draining their pastures as the snails cannot survive long dry periods. 6. Killing heavily infected sheep. 7. Destroying the eggs and manure of infected sheep. 8. Feeding infected sheep with salt and a little dry food. 9. Proper sanitary disposal and personal hygiene. 10. Proper disinfection of watercresses and vegetation before consumption.

SCHISTOSOMA OR SPLIT BODY Gr.: Schistos means divided and soma means body

ONLY TREMATODE TO LIVE IN THE BLOOD STREAM OF WARM BLOODED ANIMALS HENCE THE NAME BLOOD FLUKE. Causes SCHISTOSOMIASIS OR BILHARZIASIS. Schistosoma was discovered by Bilharz in 1851. MORPHOLOGY: Schistosoma has an elongated, slender body well adapted to live in the narrow blood vessels. It is the only fluke which is dioecious i.e. sexes are separate. Males are shorter and stouter than females. Lateral margin of the male body is folded ventrally into a groove for much of its length forming a gynaecophoric canal in which the females are held. (It was this groove that gave the

name SCHISTOSOMAS OR SPLIT BODY). Gynaecophoric canal is not usually long enough to enclose the female so loop of female body can be generally seen extending from the canal. At the anterior end of the body is the mouth surrounded by oral sucker. Acetabulum or ventral sucker is situated on a short projection from the body at the anterior end. The ventral sucker is larger and more muscular in the male and serves to maintain the position of the worm within the blood vessels. The suckers are armed with delicate spines. Unlike the liver fluke, Schistosoma lack a muscular pharynx (Apharyngeate). Just behind the acetabulum is the genital pore. Excretory pore is present ventrally at the terminal end of the body (Fig. 21). The intestinal caecae reunite behind the ventral sucker to form a single caecum. The length of reunited intestine varies in different species. The cuticle of males is provided with numerous minute papillae. Presence or absence of male is one of the most significant features affecting the functioning of female. If the female develops in the absence of sexually mature males, ovary is small and ova lacks cortical granules. It is possible that the female needs contact with the male to absorb amino acids from it which is required in turn for the production of normal cortical granules. Another effect of the absence of male is the inability of vitelline cells to mature because of the absence of metabolites required to trigger the maturation of these cells. Adult lives in the lumen of portal veins and its radicles of warm blooded animals normally in pairs. Three species of Schistosoma are known to inhabit man. S. japonicum or oriental blood fluke inhabits the small veins of portal system and mesenteric veins draining the ileo- cloacal region. S. mansoni inhabits the small branches of mesenteric veins of the rectal area and branches of portal veins. S. haematobium inhabits the blood vessels of bladder and urinary tract. All the three species of Schistosoma are morphologically similar and their life- cycle is also the same. They differ in their habitat. Hence, only Schistosoma japonicum will be described here. Schistosoma japonicum

COMMON NAME : ORIENTAL BLOOD FLUKE GEOGRAPHICAL DISTRIBUTION : Japan, Korea, Philippines, Thailand, Cambodia, Celebes, China, parts of Burma etc. DEFINITIVE HOST : Man INTERMEDIATE HOST : Fresh water snail–Oncomelania sp. RESERVOIR HOST : Pigs, Dogs, Cats, Cattle, Goats, Horses and Rodents LIFE CYCLE: MECHANISM OF EGG LAYING AND EXPULSION OF EGG: Adult blood flukes live chiefly in the superior mesentric veins, capillaries of last part of ileum, caecum, ascending colon and rectal plexus of vein. Copulation takes place while the female is held in the gynaecophoric canal of the male. After copulation, the females held in the gynaecophoric canal of males extend their anterior end far into the smallest venules or leave the male to lay eggs in small venules of mesentries of intestinal wall. Eggs are deposited longitudinally one at a time. Each time an egg is laid; worm withdraws a short distance and lays another egg immediately behind the first. In this way venules are filled with eggs pointing

backwards. The worm then migrates to adjacent venules. Eggs are held in position by the lateral knob and also by the contraction of vessels resulting from, the withdrawl of parent worm. When the vessels are laden with eggs, they rupture – Masses of eggs cause pressure on the thin venule walls which are further weakened by secretions of histolytic gland of the miracidium larva within the eggs.

VENULE WALL RUPTURES EGGS penetrate the intestinal lumen making their

URINARY BLADDER way through the vessels and mucosa

ESCAPE WITH URINE OR FECES

In heavy infestation, thousands of worms may be present in blood vessels.

EGGS when voided are EMBRYONATED AND NON-OPERCULATED.

HATCHING TAKES PLACE IN H2O The factors responsible for hatching, i.e. for the emergence of miracidium larva and its penetration into the intermediate host are same as that for liver fluke. Total life span of miracidium within an egg is 20 days after which miracidium degenerates. The life-cycle resembles that of liver fluke.

DIMORPHIC ADULT EGG MIRACIDIUM ENVIRONMENT TAKES 4-8 WEEKS TO TRANSFORM INTO TUBULAR SPOROCYST MAN SNAIL SPOROCYST

CERCARIA DAUGHTER SPOROCYST

Development takes place in the digestive gland of SNAIL REDIA AND METACERCARIA ARE ABSENT. The structure of miracidium and sporocyst larva is also the same as that of liver fluke miracidium and sporocyst larva. CERCARIAE: Structure is same as that of cercaria larva of liver fluke except that the Schistosoma cercariae have a bifid tail and instead of cystogenous glands they have a pair of penetration glands at the anterior end. The cercaria of Schistosoma is known as furcocercaria because of the presence of forked tail. Infection results when human beings bathing or walking in water come in contact with cercaria larva. The cercaria larva locates its prey purely by chance. When cercariae comes in contact with the skin of an appropriate host, they loop for variable periods of time by attaching themselves alternately with the oral and ventral suckers. When unattached, the oral end of the body constantly probes into every irregularity encountered. Points of entry include wrinkled areas, bases of follicular eminences, points of scale attachment, distal hair, entry site used by previous cercariae. These cercariae burrow in the skin with the help of penetration glands and

body movements. Eventually, cercariae become closely attached by their oral sucker and assume a vertical or oblique position in relation to the surface. Penetration glands release an enzyme probably hyaluronidase (which hydrolyses hyaluronic acid – one of the principal substrate of connective tissue); although the exact nature of the secretion is not known. Ramming motion and partial evertion of sucker brings the ducts of penetration gland in contact with the skin. Through these ducts, gland secretions are poured into the host tissue. Alternate contraction and elongation of the body and energetic movement of tail, thrusts the oral sucker deeper.

After penetrating the skin of the definitive host, cercariae cast off their tail to become a schistosomule and gain access to peripheral venules. From here they are carried through the right heart into the pulmonary capillaries. It takes some days for the larva to pass through the capillary bed in the lungs. Then they are carried through the left heart into the systemic circulation. Majority of them are shunted into the abdominal aorta and gain access to the mesenteric artery, pass through capillary bed in the intestine and enter portal cirulation ~ taking 5 days to reach the liver. The schistosomule mature into adults in about 3 weeks in intrahepatic portal blood stream. Mature adults then move out of the liver and enter the superior mesenteric vein from where they finally settle down in the capillaries of the last part of the ileum, caecum and colon. SCHISTOSOMULE FROM THE PERIPHERALVENULES OF SKIN RIGHT HEART

LEFT HEART PULMONARY CAPILLARIES It takes some days for the larva to pass through the capillary bed in the lung

SYSTEMIC CIRCULATION

LIVER Schistosomule mature into adults in about 3 weeks in intrahepatic portal blood stream

MATURE SCHISTOSOMAS SUPERIOR MESENTERIC VEIN MOVE OUT OF LIVER AGAINST BLOOD CURRENT A month or more elapses from the time of penetration by the cercariae to the schistosomas eggs in feces.

FEMALE ENTERS GYNAECOPHORIC CAPILLARIES OF LAST PART CANAL OF MALE OF ILEUM, CAECUM AND COLON

LIFE SPAN OF ADULT WORM IS 5-30 YEARS Several thousand individuals may exist in a single host. Schistosoma infection occurs much frequently in : • Laboureres working in irrigated fields. • Fishermen working in fish culture ponds and river. • Women who wash utensils or clothes along the banks of canal or river PATHOGENICITY: Schistosoma causes SCHISTOSOMIASIS or BILHARZIASIS in man.

SEVERE FROM OF DISEASE MAY CAUSE

ENLARGED CALCIFIED DEFORMITY MALFUNCTIONING LIVER OR BLADDER OR IN URETER OR OF KIDNEY

Antigens from the parasite induces the host to produce antibodies which results in antigen- antibody reaction which is responsible for early clinical symptoms. PATHOGENICITY OF S. JAPONICUM • Causes intestinal and hepatic schistosomiasis of orient also known as katayama disease. Lesions produced are pronounced because of large output of eggs. Ileo- cloacal region is affected causing dysentery. KATAYAMA DISEASE SYMPTOMS – Fever, Abdominal Pain, Diarrhoea and Allergic manifestations. • Gastroinstestinal bleeding, Ulceration and necrosis of intestinal tissue. • Enlargement of spleen. • Migrating worms cause little or no damage or symptoms but occasionally serious reactions occurs, such as pneumonia resulting from invasion of the lungs. • Liver invading phase may be symptomless or it may be marked by enlargement and tenderness together with some toxic reactions. • Some common symptoms are: Cough, Fever, Diarrhoea, Eosinophilia, and Anaemia. The arms, body and legs become alarmingly thin. Abdomen becomes enlarged and is filled with fluid. DIAGNOSIS: 1. Stool/ Urine examination. 2. Biopsy of infected Area. PROPHYLAXIS • Erradication of intermediate molluscan host. • Prevention of Environmental pollution with urine and feces. Developing proper sanitary disposal method. • Effective treatment of infected persons • Avoidance of swimming, bathing, wading or washing in infected water.

• Infected person should not be allowed to wade in H2o with open wounds. • Health education. Taenia solium Commonly called as pork tapeworm; the armed tapeworm of man. The life cycle of the parasite was first described by van Beneden(1854). He demonstrated the larval stage in the muscles of pig after feeding it with eggs from human faeces. Kuchenmeister(1855) demonstrated the adult tapeworm in the intestine of man.

GEOGRAPHICAL DISTRIBUTION : Worldwide in distribution. DEFINITIVE HOST : Man HABITAT : Adult worm lives in the small intestine (upper jejunum) of man. Normally only a single worm is present but rarely several worms may be present. They remain coiled up in the intestine.

MORPHOLOGY : The adult worm has a long ribbon like body measuring about 2-3metres and consists of a scolex(head), neck and strobila made up of many proglottids(sometimes, though wrongly, referred to as segments) (Fig. 22). SCOLEX : The scolex is small, quadrate about 1mm in diameter with four cup-like suckers at equatorial position and a conspicuous rounded distal bulge, rostellum, armed with double row of 25-30 alternating large and small hooklets (Fig.23 A, B). The hooks and suckers are the holdfast organs and help the animal to firmly attach themselves to the intestine of the host. NECK: The neck is short and half as thick as the head. Neck is the zone of proliferation and new proglottids are budded off from the posterior part of the neck –a process known as strobilization. As new proglottids are added on behind the scolex, the older ones are pushed away farther and farther from it during growth, so that the younger ones are nearer the scolex and older ones away from it, and oldest proglottid is the last one (Fig.22). STROBILA: The strobila consists of 800-1000 proglottids-immature, mature and gravid in that order from front backwards. The immature proglottids are the youngest, undifferentiated just behind the neck and are devoid of reproductive organs. They are about 200 in number and are broader than long. The mature proglottid is a complete reproductive unit. The anterior 100-150 mature proglottid contain only the male reproductive organs while the later 250 mature proglottid have both the male and female reproductive organs. The lateral margin of each proglottid bear alternately on the right and left side a small protuberance called genital papillae through which opens a common genital pore. The gravid proglottids(last 150-350) are the posteriormost and bear only a highly branched uterus full of fertilized eggs. All the organs except the uterus get atrophied in the gravid proglottids and they become twice as long as broad (Fig. 22). These gravid proglottids are regularly detached and expelled out passively in short chains of 5 or 6 along with the host faeces-a process known as apolysis. Apolysis limits the size of the tapeworm which may otherwise attain enormous length due to continuous proliferation in the neck region. Also, apolysis serves to transfer the developing embryos to the exterior, where they can be ingested by the intermediate host. LIFE CYCLE: DEFINITIVE HOST : Man INTERMEDIATE HOST : Pig Copulation takes place between segments of the same worm. Because of protandry, the male reproductive organs mature before the female, and thus during copulation, there is folding up of the strobila bringing the proglottids, where the male reproductive organs have matured in contact with those in which the female organs have also matured, and the sperms are transferred by the cirrus into the vagina from where they enter the fertilization duct. The ova are also discharged from the ovary into the oviduct from where they pass into the fertilization duct. Fertilization takes place and the fertilized egg then passes into the ootype where they are surrounded by yolk cells secreted by vitelline glands. The yolk cells form a thin shell known as chorionic membrane around the egg. This encapsulated egg then passes into the uterus. When a large number of capsules are collected in the uterus, it enlarges and gives off branches so that the gravid proglottids contains only the highly brached uterus full of eggs, all other organs get atrophied. The fertilized egg or zygote undergoes holoblastic and unequal cleavage and develops into three types of cells viz. micromeres, mesomeres, and megameres. Micromeres form the morula which develops three pairs of chitinous hooks from cells known as onchoblast. The morula is surrounded by inner membrane or embryophore formed by the mesomeres. Beneath the embryophore is the basement membrane (Fig. 24). The embryophore is in turn surrounded

by the outer envelope of megameres. The yolk cells and the envelope of megameres give their yolk to the developing embryo and disappear. The six-hooked embryo, called hexacanth, possesses a pair of large penetration gland and is surrounded by two hexacanth membranes. The hexacanth with all the membranes surrounding it is known as the oncosphere.

INFECTION TO THE SECONDARY HOST: Gravid proglottids containing eggs are passed out along with the feces where they degenerate setting free the oncospheres. These are ingested by the pig. Further development takes place in the body of the pig which acts as the intermediate host. In the alimentary canal of pig, the oncosphere looses its embryophore and basement membrane by the action of the acidic juices of the stomach and hexacanth passes into the intestine where the two persisting hexacanth membranes are also lost by the action of alkaline juices. Activated by the bile salts, the hexacanth penetrates the intestinal wall with the help of hooklets and penetration glands. With the help of the hooks the hexacanth anchors itself to the intestinal wall while the secretions of the penetration glands dissolves the intestinal tissues and the embryo gains entry into the portal vessels or mesenteric lymphatics and are carried in systemic circulation to different parts of the body. Finally they are filtered out in the striated muscles usually of the tongue, shoulder, neck, thigh etc. where in 10-12 weeks they develop into a larva known as cysticercus cellulosae. However, they may develop in other organs such as liver, lung, kidney or brain. The hooks are no longer of any use and are shed off. The cysticercus cellulosae or bladder worm is an ovoid, opalescent (milky white) bladder surrounded by a fibrous capsule. It has an invaginated scolex with four suckers and a rostellum with a double row of alternating large and small hooklets (Fig. 25). It absorbs nourishment from the hosts tissue. The bladder contains a thick fluid rich in protein and salt. In fact the fluid within the bladder is largely composed of hosts blood plasma. The cysticercus usually measures about 5mm-10mm and lies parallel to the muscle fibres. It can be seen as a thick white spot. The pigs flesh which is infested with cysticercus is known as measly pork. The bladder worm can remain viable for several months. The development time of cysticercus cellulosae is 10-12 weeks in pig.

Cysticercus cellulosae can develop into an adult tapeworm only when ingested by man. The cysticercus larva remains dormant in the muscles or connective tissue of pig without any further development, till it happens to be eaten by man.

INFECTION TO THE PRIMARY HOST: Man acquires the infection by consuming inadequately cooked pork (measly pork) containing cysticercus cellulosae. The larvae are digested out of the meat in the duodenum. The scolex evaginates on coming in contact with the bile and anchors to the mucosal surface by means of their suckers. The neck begins to proliferate forming a chain of proglottid and an adult tapeworm is formed in 5-12 weeks (Fig. 25).

A man harbouring the adult worm may autoinfect onself by reverse peristaltic movement of the intestine whereby the gravid proglottids are thrown into the stomach and oncospheres are liberated. The total life span of the adult worm is very long- about 25 years or may be more. PATHOGENESIS: Adult worm in the small intestine donot cause any harm apart from vague abdominal discomfort, chronic indigestion, persistent diarrhoea alternating with constipation, anaemia, weight loss and nervous discomforts. Hooks and suckers may cause mechanical irritation in the intestine, which may initiate reverse peristalsis leading to autoinfection. The symptoms caused by the adults may be referred to as taeniasis. It is the larval stage that cause serious trouble and the disease is known as cysticercosis. Cysticercus cellulosae usually occur in large numbers but sometimes may occur singly and affect any organ or tissue. It may also affect the eyes, brain, and less often the heart, liver, lungs, abdominal cavity and spinal cord. The effect produced depends upon the site affected. They usually develop in the subcutaneous tissues and muscles forming visible nodules. The larva evokes a cellular reaction, with infiltration of neutrophils, eosinophils, lymphocytes, and plasma cells. This is followed by fibrosis and death of the larva with eventual calcification. In cysticercosis of the brain (neurocysticercosis), symptoms are more often due to dead and calcified larvae rather than the living larvae. Epilepsy is the commonest manifestation, but it can also cause behavioural disorders. Ocular cysticercosis may cause blurring of vision and ultimately blindness. Diagnosis of cysticercosis can be carried out by: 1. Biopsy of subcutaneous nodule which may reveal cysticerci 2. X-ray of skull and soft tissue may reveal calcified cysticerci. 3. CT scan of brain can accurately locate the lesion in the brain. 4. Ocular cysticercosis can be made out by ophthalmoscopy. DIAGNOSIS: Stool examination for the eggs. PROPHYLAXIS: 1. Personal hygiene and proper sanitary disposal. 2. Strict inspection of slaughter houses with condemnation of measly pork(infected meat). 3. Thorough cooking of pork. 4. For control of cysticercosis, prevention of faecal contamination of soil, proper disposal of sewage and avoiding eating of raw vegetables grown in polluted soil. 5. Persons harbouring adult worms should be isolated and treated as they can develop cysticercosis due to autoinfection.

Diphyllobothrium latum Commonly known as fish tapeworm or the broad tapeworm.

Greek: diphyllobothrium means having two grooves in the head; latus means broad (di means two; phyllon means leaf and bothrium means sucking organs i.e. a leaf-shaped structure having sucking organs). Although the head of the worm was found as early as 1777 by Bonnet, the life cycle was worked out by Janicke and Rosen in 1917. GEOGRAPHICAL DISTRIBUTION: Endemic in central and northern Europe, Japan, Central Africa, Russia and North America. So far only one case of human infection has been reported from Vellore in India in 1998. Till then India was free of the infection by this parasite. DEFINITIVE HOST: Man. Also found in dogs, cats and their other wild relatives feeding on fishes. HABITAT: Adult worm lies folded in several loops in the small intestine, usually in the ileum of the definitive host. MORPHOLOGY: It is the longest tapeworm found in man, measuring upto 10 metres or more in length. A freshly expelled worm from human intestine is ivory colored and consists of a scolex (head), neck and strobila. SCOLEX: It is spoon shaped, about 2-3mm long and 1mm broad. It bears two slit –like longitudinal sucking grooves (bothria), one dorsal and the other ventral (hence the name). Rostellum and hooklets are absent. NECK: Behind the scolex is an unsegmented thin neck, several times longer than the scolex. Neck is the zone of proliferation i.e. new proglottids are budded off posteriorly from the neck so that the youngest proglottids are immediately behind the neck. STROBILA: The strobila may have 3000 or more proglottids (often incorrectly called as segments) consisting of immature, mature and gravid proglottids. A mature proglottid is broader than long and contains male and female reproductive organs. Three genital openings are present ventrally along the midline- the openings of the vas deferens, vagina and uterus. LIFE CYCLE: DEFINITIVE HOST: Man, and other freshwater fish eating animals like dogs, cats, foxes and wolf etc. INTERMEDIATE HOST: D. latum is unique among human tapeworms because they require two intermediate host to complete its life cycle. FIRST INTERMEDIATE HOST: Small copepods mainly Cyclops. SECOND INTERMEDIATE HOST: Freshwater fishes like salmon, trout, perch etc. D. latum is a prolific egg laying worm and a single worm can discharge about a million eggs in a day. The terminal proglottids become shrunken because of the constant discharge of the eggs. Later these proglottids get dried-up and finally break off from the body in chains and voided out along with the faeces in water. EGGS: Eggs when voided out are oval in shape, yellowish brown in color (bile stained), 70µm long and 45µm broad with a thin smooth shell. The eggs bear an operculum at one end and a small knob at the other end. The eggs are resistant to chemicals but are readily killed by drying.

Further development of the eggs takes place in fresh water lakes, rivers or reservoirs. Freshly passed eggs are not infective to man. Within 1-2 weeks a ciliated embryo containing three pairs of hooklets ( hexacanth embryo) develop within each egg shell. It emerges out through the operculum and is known as the coracidium. It is the first stage larva, spherical in shape and bears cilia. Coracidium larva swims about in water but can survive for only 12 hours in water by which time it must be ingested by an appropriate copepod- Cyclops (crustacean) which is the first intermediate host. In the midgut of the cyclops, the coracidium sheds off its cilia and with the help of the six hooklets penetrates the midgut wall and gains entry into the haemocoel (the body cavity). In the haemocoel of cyclops, it gets transformed into a second stage larva known as procercoid larva which is spherical, 550µm long with a caudal appendage known as cercomer that bears the hooklets which are of no more use. If this infected copepod (cyclops) is now eaten by a suitable fresh water fish (the second intermediate host), the procercoid larva penetrates the intestine of fish and migrates to the muscles, liver and fat of the fish within a few hours and grows. It looses its caudal appendage and develops into a third stage larva- the plerocercoid larva or sparganum (Fig. 26). The plerocercoid is a flattened, unsegmented, glistening white larva measuring 10-20mm in length with a rudimentary invaginated scolex. This is the infective stage for man. Humans become infected when they eat undercooked, or raw freshwater fish infected with plerocercoid larva. In the intestine of man, the plerocercoid larva develops into an adult and attains maturity in about 5-6 weeks and begins to lay eggs which are voided out along with the faeces. The cycle is then repeated. An adult worm can live for about 10 years or more. PATHOGENICITY: The pathogenic effects of diphyllobothriasis depend on the mass of the worm, absorption of its by-products by the host and deprivation of the hosts metabolic intermediates. In some persons, the infection may be asymptomatic while in others the worm may cause discomfort, diarrhoea, nausea, weakness and numbness of the extremities. Some patients develop mechanical obstruction of the bowel because of the presence of a large number of worms. In a few cases, pernicious anaemia called bothriocephalus anaemia may develop due to manifest vitamin B12 deficiency. D. latum adult has a great affinity for vitamin B12. It can absorb as much as 80- 100% of a single dose of vitamin B12, thereby competing with the host for this important vitamin. DIAGNOSIS: Stool examination for eggs or proglottids. PROPHYLAXIS: 1. Thorough cooking of freshwater fish. 2. Freezing of fish intended to be eaten raw at -18ºC for 24-48 hours kills plerocercoids. 3. Preventing the contamination of lakes, ponds and river water by human faeces. 4. Effective sanitary disposal of faeces. 5. Protection of water supplies from faecal pollution.

Chymenolepis nana commonly known as the dwarf tapeworm. The smallest and the commonest tapeworm found in the intestine of man. It derives its name from the Greek word: hymen means a membrane and lepis means rind or covering; and nana in Greek means a small size (nanus- dwarf). It was first discovered by Bilharz in 1857.

GEOGRAPHICAL DISTRIBUTION: A cosmopolitan worm but more common in warm climates. H. nana infection is prevalent throughout India. DEFINITIVE HOST: Man and rodents like mice and rats. In rodents, the tapeworm is regarded by a different strain- H. nana var. fraterna. The murine strain does not appear to infect man but the human strain may infect rodents, and therefore, constitute a reservoir of infection for the human parasite. HABITAT: In man the adult worm is found in the upper two third of the ileum in large numbers while in rodents they reside in the posterior part of the ileum. MORPHOLOGY: It is the small, thread like worm measuring only 5-45mm in length and 1mm in diameter. Like other tapeworms, the body consists of scolex, neck and strobila. SCOLEX: The scolex is globular with 4 cup- shaped suckers and a retractile rostellum armed with a single row of hooklets. NECK: A long and slender neck is situated posterior to the scolex. STROBILA: It consists of 200 or more proglottids. They are much broader than long. In an infected person as many as 1,000-8,000 worms may be present. LIFE CYCLE: It is a unique tapeworm requiring only one host to complete its life cycle. Eggs and proglottids are voided out along with the faeces. The eggs are spherical or ovoid, measuring about 30-45µm in diameter with a smooth, thin colorless outer membrane and an inner embryophore enclosing the hexacanth embryo (oncosphere) with 3 pairs of hooklets. There is a clear space between the outer and inner membrane which is filled with yolk granules. Man acquires the infection by ingesting the eggs via fecal-oral route( by consuming food and water contaminated by H. nana eggs or by ingesting eggs from contaminated hands). Rarely, man can acquire infection by ingestion of food contaminated with fleas harbouring the cysticercoid larvae (Fig. 27). When the eggs are swallowed, they hatch in the lumen of the small intestine(duodenum or jejunum) and a small hexacanth embryo is liberated. The hexacanth now penetrates the jejunal villus and develops into a cysticercoid larva in about 4 days. The villus then ruptures and cysticercoid larva becomes free in the lumen of the small intestine. The cysticercoid is a solid pyriform structure, with an invaginated scolex and a short conical posterior end. In the lumen of the small intestine, the mature larva evaginates its scolex and attaches itself to the mucosa by its suckers (Fig. 27). It starts strobilization to become a mature worm and in about 30 days after the infection of eggs, the proglottids begin to appear in faeces and the cycle is repeated. Internal autoinfection may also occur when the eggs released in the intestine instead of being voided out along with the faeces hatch in the lumen of the intestine itself. Life span of adult worm is about two weeks. H. nana also has an indirect life-cycle with insect as an intermediate host. The insects as intermediate host include flour-eating beetles like the species of Tribolium and Tenebrio, fleas such as Xenopsylla cheopis, Pulex irritans and Ctenocephalides canis. The adult insects or their larvae eat the eggs of H. nana and reach the gut of the insect. In the lumen of the gut, the enzymes stimulate the oncosphere to free itself from the enclosing membrane. The oncospheres with the help of the six hooklets and the glandular secretions penetrate the gut wall and enter the haemocoel of the insect where they transform into a cysticercoid larva, which is infective to

the final host. Man gets the infection by accidentally ingesting these insects. In the intestine of man the cysticercoid larva develops into an adult worm. Rodents get infected when they eat these insects. PATHOGENICITY: Infection with Hymenolepis does not generally produce any illness. Symptoms may sometimes occur due to allergic reactions. Some of the symptoms are: abdominal discomfort, headache, diarrhoea, restlessness, dizziness, sleep disorder and anorexia. DIAGNOSIS: Stool examination for the presence of eggs. PROPHYLAXIS: 1. Proper personal hygiene. 2. Sanitary improvement. 3. Protection of water supplies from faecal contamination. 4. Avoid eating flour or rice infested with insects.

Echinococcus granulosus Commonly known as dog tapeworm or the hydatid tapeworm. In man it causes the disease unilocular echinococcosis or hydatid disease. Adult E. granulosus was first described by Hartmann(1695) in the intestine of dog. The life cycle was demonstrated by Von Siebold(1853). Naunyn (1863) demonstrated that the hydatid cyst in man is the bladder worm stage of Echinococcus found in the intestine of man.

GEOGRAPHICAL DISTRIBUTION: World-wide but is more prevalent in sheep and cattle- raising countries. In India, a large number of cases have been reported from Andhra Pradesh, Gujarat, Tamilnadu, West Bengal, Orissa, Bihar, Punjab, Uttar Pradesh and Pondicherry. DEFINITIVE HOST: Dog and other canine carnivores. INTERMEDIATE HOST: Sheep, goat, cattle, pig, horse and man. HABITAT: The adult worm lives in the jejunum and duodenum of dogs and other canine carnivores with its scolex buried in the mucosa. A large number of them may be present in the infected dogs. MORPHOLOGY: It is a small tapeworm measuring 3-6mm in length. The body consists of a scolex, a short neck and strobila composed of only 3 proglottids (occasionally four). The first proglottid is immature, the second one is mature and contains both the male and female reproductive organs while the third one is the gravid proglottid containing a highly branched uterus full of eggs. The scolex is pyriform in shape with four suckers and a protrusible rostellum with two circular rows of hooklets. LIFE- CYCLE: E. granulosus completes its life cycle in two host. The adult lives attached to the mucosa of the small intestine of dogs and other canine animals. The eggs are discharged along with the faeces of the definitive host. Sheep and other intermediate host get infested by consuming these eggs while grazing in the field. Man is an accidental host. EGGS: The eggs are spherical, brown in color, measuring 31µm-43µm in diameter. The egg is made up of two layers- an outer thin layer and an inner embryophore. Within the embryophore lies a hexacanth embryo with 3 pairs of hooklet. Within eight hours of ingestion, the hexacanth embryo hatches out in the duodenum of the intermediate host and penetrates the intestinal wall to enter the radicles of the portal vein from where they are carried to the liver. The liver acts as the first filter for the embryos where they get arrested in the sinusoidal capillaries. Some of the embryos escape and gain entry in the pulmonary circulation. Lungs now act as the second filter. However, some of the embryos escape even the pulmonary circulation and enter the general circulation from where they lodge in various organs like heart, brain, spleen, kidneys, bones, muscles, etc. At the site of deposition, an active cellular reaction takes place around the parasite and a large number of them are destroyed by the hosts defense mechanism. Some of the embryos that escape destruction gets surrounded by fibrous tissue which is known as pericyst (Fig. 28). This pericyst merges with the normal surrounding tissue. The parasite derives its nutrition through this layer. Inside this pericyst, the embryo develops into a fluid filled bladder or cyst known as hydatid cyst (Greek: hydatis means a drop of water). The hydatid cyst is composed of two layers: an outer laminated ectocyst and an inner germinal endocyst. From the inner layer of the cyst, brood capsules with a number of scolices develop. The inner layer also secretes the hydatid fluid and gives rise to the outer layer. In a few cysts, brood capsules fail to develop or even if they develop, they donot contain any scolices. Such cysts are called sterile or acephalocyst. These acephalocyst if ingested by the definitive host donot cause any infection. When sheep or cattle harbouring hydatid cyst die or are slaughtered, dogs may feed on the carcass and get infected. When the hydatid cyst reaches the intestine of dogs, the cyst wall ruptures, scolex evaginates and develops into an adult. The adult attains maturity in 6-7 weeks and produce eggs to repeat the cycle. This is the natural cycle of the host. However, if man gets the infection by direct contact with the dogs, or by consuming water and food contaminated with the dogs faeces containing eggs of Echinococcus, then the life-cycle of the parasite comes to a dead end as the human hydatid cyst are unlikely to be consumed by the dogs.

PATHOGENECITY: Infection is normally acquired during childhood when intimate contact with pet dogs is more likely. However, the clinical symptoms develop several years later. In majority of cases, the primary hydatid occurs in the liver. Hence, hepatomegaly, pain and obstructive jaundice are the usual manifestations. When the cyst enters the lung, the symptoms are: cough, chest pain and dyspnoea. In the kidney it causes pain and haematuria. Erosion of bones may lead to pathological fractures and degeneration of bone structures. Hydatid cyst of the spleen, heart and brain may present a tumour-like condition or abscess. DIAGNOSIS: Ultrasonography and CT scan in most of the cases can reveal the diagnosis. PROPHYLAXIS: 1. Strict personal hygiene. 2. Dogs should not be allowed to eat the carcasses of slaughtered animals in endemic areas. 3. Reduction of stray dog population has been found to be helpful. 4. Periodical deworming of the pet dogs is useful. 5. Kissing of pet dogs should be discouraged.

EVOLUTION OF PARASITISM IN HELMINTHES

It is believed that the parasitic platyhelminthes have been derived from the free living turbellarian progenitors which were ectoparasites on molluscan, crustacean and echinoderm host. According to Hyman, mollusk could have been the original hosts for digenetic trematodes. Some ancestral rhabdocoels (Turbellarians) with a suctorial pharynx could have entered the mollusc body accidentally while feeding on the soft tissues. The adults may have left the mollusk body to lay eggs. With the evolution of vertebrates, they invaded the new hosts but the

connection with the original host retained which then became the intermediate host. During the course of evolution, the endoparasites acquired certain adaptations for their survival in the body of the vertebrate host. These adaptations were both morphological and physiological. Thus, a vertebrate or the second host became obligatory for these endoparasites. Even a third intermediate host was incorporated in the life cycle of these endoparasites. The disadvantages of two or three hosts may have been compensated for by the development of polyembryony and larval multiplication in the molluscan host. This may have led to the evolution of the modern digenean endoparasites with their complicated life-cycle. The monogenean parasites of vertebrates probably arose from the free living rhabdocoels long after the digenean parasites. The ancestral monogeneans probably fed on the skin of the sluggishly moving early vertebrates just like the turbellarians, and developed a haptor which allowed them to maintain a permanent association with their food supply. Generally the monogeneans are ectoparasites but some of them may have invaded the bladder, intestine and coelom of their host e.g. Gyrocotyle. Living in a medium of pre-digested food, the parasite lost its gut while retaining its characteristic monogenean features of posterior haptor and a direct single host life-cycle. The Gyrocotylidean-like monogeneans may have been the ancestors of cestodes. PARASITIC ADAPTATION IN HELMINTHES Parasitic adaptation may be defined as morphological, physiological and behavioural modifications developed by the organism to lead a parasitic mode of life. MORPHOLOGICAL ADAPTATIONS: Morphological adaptations can either be: ¾ Loss of organs OR ¾ Attainment of new organs LOSS OF ORGANS: ¾ LOCOMOTION: Locomotion is generally needed by the animal to procure food and shelter. As Fasciola is an endoparasite, it does not need to make excursion for food and safety. Hence, the locomotory organs are absent. However, locomotory organs are present in the free living larvae of the parasitic forms. For example, the miracidium larva of Fasciola has cilia while the cercaria larva possesses a tail for locomotion. ¾ DIGESTIVE SYSTEM: As the endoparasites feed on the digested or semi-digested food of the host, the digestive system is either absent as in class cestoda or is greatly simplified as in Flukes. Trematodes eg. Liver fluke have an incomplete gut with a single opening, the mouth; anus is absent in them. Since the food is already in the digested state, there is complete absence of digestive glands. Cestodes like tapeworm directly absorb the hosts digestive juices through the tegument and hence there is a complete absence of the alimentary canal. ¾ CIRCULATORY AND RESPIRATORY SYSTEM: Absent ¾ EXCRETORY SYSTEM: Excretory system is quite efficient to remove the excretory wastes so that proper metabolic activities may be maintained. ¾ NERVOUS SYSTEM: Nervous system is poorly developed and sense organs are lacking. Since Fasciola is an endoparasite it does not react with the external environment and the internal environment of the host is more or less uniform, hence the sense organs are absent in them. ATTAINMENT OF ORGANS:

¾ Dorsoventrally flattened body. Flat and thin body enables helminthes to live in narrow spaces. ¾ ORGANS FOR ADHESION: Adhesive organs like suckers in Fasciola and hooks and suckers in Taenia help the parasite to attach itself to the hosts tissues. ¾ BODY COVERING: Tough and resistant tegument is an important parasitic adaptation. The body covering is frequently provided with scales and spines which afford suitable protection to the parasite. The tegument also provides protection against the action of digestive juices of the host. ¾ Fasciola has a muscular and suctorial pharynx, adapted to suck the hosts digestive juices. ¾ REPRODUCTIVE SYSTEM: The internal parasites are characterized by a complicated reproductive system designed and perfected to meet the need for the tremendous egg production. Almost all parasitic helminthes are monoecious except Schistosoma. Hermaphroditism is of distinct advantage to the parasite as: • It ensures copulation even when few individuals are present. • After copulation both individuals lay eggs, thus doubling the rate of production. • In the absence of another member of the same species, the parasite can reproduce offspring by itself, like in Taenia. ¾ In event of the failure of cross fertilization, they resort to self fertilization in which close proximity of the cirrus and vulva is of great help. ¾ In flatworms, the ovaries and testes are either greatly enlarged or show an increase in number so that they are capable of producing a large number of gametes. In cestodes, the reproductive system is much more elaborate and each mature proglottid possesses one or two sets of male and female genetilia. In a gravid proglottid all other organs of the system degenerate to make room for the highly branched uterus. PHYSIOLOGICAL ADAPTATIONS: ¾ Tegument is highly permeable and the helminthes can absorb the hosts digestive juices by the tegument by simple diffusion. ¾ These endoparasites are facultative anaerobes. They respire by the breakdown of glycogen. ¾ They are well adapted to the hosts osmotic concentration and since their osmotic concentration is same as that of the host they don’t need to osmoregulate. However, in the intestinal tapeworms the osmotic pressure is a little higher. This permits the ready absorption of the hosts digested juices by the parasite. ¾ They have high pH tolerance. ¾ Parasites secrete antienzymes to neutralize the digestive enzymes of the host. ¾ These parasites have a complicated life cycle. They require two or more host to complete its life cycle. A very large number of eggs are produced by helminthes-10,000 or even more. Such an enormous number of egg production is needed because the chances of survival of the eggs are less. Rate of reproduction of an organism is directly proportional to the chances of death it faces at various phases of its life cycle. In flukes, a single egg produces several embryos, a process called polyembryony. In them, the various larval forms are produced by the simple asexual multiplication of germ cells. Thus, a single zygote gives rise to several adults. ¾ The eggs are covered by a resistant capsule or shell as a result of which they can remain viable for a long time even in adverse conditions.

¾ Involvement of several larval forms in the life cycle ensures transmission of parasite from one host to another. Intermediate host ensures wide dispersal of the species. ¾ Parasites have successfully solved the problem of extinction by finding out suitable reservoirs in which they thrive but never cause any harm to them. CLASSIFICATION: Phylum Platyhelminthes is divided into four classes of which only class turbellaria include free living forms while the remaining three classes include parasitic forms: CLASS TURBELLARIA CLASS MONOGENEA CLASS TREMATODA CLASS CESTODA Classification has strictly been followed from Invertebrate Zoology by Robert D. Barnes(5th edition) CLASS TURBELLARIA: (L., turbella means a stirring) 1. Mostly free living either terrestrial or aquatic found in both fresh and marine water. Most aquatic forms are bottom dwellers living in sand or mud, under stones and shells, or on sea weed. 2. Broad, flat leaf-like soft body covered with a cellular or syncytial epidermis having secretory cells and rhabdites. The epidermis is partly ciliated. 3. Reproduction is either asexual or sexual. Asexual reproduction is by fission or by strobilization. 4. Mostly hermaphrodite. Simple life cycle. Development is direct in most turbellarians, however some polyclads produce a free swimming larva. 5. They have a great power of regeneration. Despite their similarity in appearance, turbellarians exhibit considerable internal complexity, and the class is composed of a relatively large number of diverse groups. Turbellarians are further grouped into two: 1. Archoophoran 2. Neoophoran Archoophoran Turbellarians: 1. They reflect a more primitive level of organization. 2. Yolk glands are absent. 3. Entolecithal eggs. 4. Spiral cleavage. ORDER : Acoela 1. Small marine flatworms usually less than 2mm in length. 2. Gut without pharynx and devoid of a cavity. 3. Protonephridia absent. 4. Gonads are often not bounded by a cellular wall. 5. A few species ate commensal living in the intestine of echinoderms. Example: Anaperus, Convoluta, Afronta ORDER : Nemertodermatida 1. A small group with digestive tract lined by epithelial cells.

2. Small marine species similar to acoels but possessing uniflagellate sperm. Example: Nemertoderma ORDER : Macrostomida: 1. Small, freshwater and marine species with a simple pharynx and a simple saclike ciliated intestine. Example: Microstomum, Macrostomum ORDER : Haplopharyngida 1. Small marine animals similar to macrostomida but possessing a proboscis and a temporary anus. Example: Haplopharynx ORDER : Catenulida 1. Small freshwater species having a simple pharynx and a ciliated saclike intestine. 2. Gonads are unpaired. 3. Male gonopore is dorsal above the pharynx. 4. Female gonoduct absent. Example: Stenostomum, Catenula ORDER : Polycladida: 1. Marine flatworms of moderate size, with a flattened more or less oval body. 2. A pair of anterior marginal or dorsal tentacles may be present. 3. Muscular pharynx present. 4. Intestine is centrally located, elongate with many branched, radially arranged diverticula. 5. Numerous eyes. 6. Many of them are brightly colored. Example: Stylochus, Notoplana, Leptoplana Neoophoran Turbellarians: 1. They reflect an advanced level of organization. 2. Yolk glands present. 3. Ectolecithal eggs. 4. Development greatly modified from the spiral pattern. ORDER : Prolecithophora: 1. Usually small, freshwater and marine species having a bulbous pharynx and a simple intestine. 2. Ovary produces eggs and follicle-like egg cells. Example: Plagiostomum ORDER : Lecithoepitheliata 1. Freshwater and marine species with a simple intestine.

2. Mouth and complex pharynx is situated at the anterior end. 3. Ovary produces eggs surrounded by follicle- like yolk cells. Example: Prorhynchus ORDER : Rhabdocoela 1. A large group of small freshwater and marine species having a bulbous pharynx and a simple intestine and a pair of nerve cord. Example: Gnathorhynchus ORDER : Proseriata 1. Small, mostly marine with some interstitial species. 2. Tubular pharynx with a simple unbranched gut. Example: Nemertoplana ORDER : Tricladida 1. Relatively large, freshwater, marine and terrestrial turbellarians with a tubular pharynx which is posteriorly directed. Gut has three branches. Example: Freshwater species include Dugesia, Procotyla etc. Marine species like Bdelloura is a commensal on the book gills of horse shoe crabs. Land planarians include Bipalium, Orthodemus, Geoplana GROUP FLUKES: They include both the classes Monogenea and Trematoda and contain over 8000 species of ectoparasites and endoparasites. Majority of them are parasites of vertebrates, especially fish, but immature stages are harboured by invertebrates. Unlike the turbellarians, the body of the flukes is covered by a non-ciliated cytoplasmic syncytium, the tegument. Adhesive suckers are usually present around the mouth and may also be present midventrally. Rhabdites are absent. CLASS MONOGENEA: 1. They have a single host in their life-cycle with only one generation i.e. one egg produces one adult. 2. Largely parasitic on marine and freshwater fishes, amphibians, reptiles, and cephalopod mollusk. 3. Majority are ectoparasite, but some of them invade the body cavities like the mouth, gill chambers and urinogenital tract. 4. Since they are attached to the skin of fast moving host, the monogeneans have a dorsoflattened body. 5. These monogenetic flukes possess a large posterior attachment organ called opisthaptor which bears hooks and suckers allowing the parasite to cling on to the host. Example: Polystoma, Gyrodactylus CLASS TREMATODA: (Gr., tremta means a hole, and eidos means form) 1. They are ecto- or endoparasitic worms. 2. Unlike the monogenea, the trematodes require two to four host to complete its life cycle. 3. Unsegmented, dorsoventrally flattened leaf like body covered with a thick cuticle.

4. Ciliated epidermis with rhabdites absent. 5. Bears suckers for attachment to the body of the host. 6. Hermaphrodite. 7. Life cycle is either simple or complex with one or more larval stages. Example: Fasciola, Schistosoma CLASS CESTODA: (Gr., kestos means girdle and eidos means form) 1. Endoparasites living in the intestine of vertebrates, commonly known as tapeworms. 2. Pseudosegmented , ribbon like body covered by a syncytial tegument. 3. Body is divided into a few to many proglottids(not true segments). 4. Ciliated epidermis with rhabdites absent. 5. Suckers and hooks are the organs of attachment. 6. Digestive system absent. 7. Hermaphrodite. Each segment is a hermaphrodite with one or two sets of male and female organs. 8. Complicated life cycle usually involving two or more hosts. SUBCLASS : Eucestoda 1. The great majority of cestodes belong to the subclass Eucestoda and are commonly known as tapeworms. 2. They posses a long ribbon-like body divided into scolex, neck and strobila with many proglottid(polyzoic). Example: Taenia, Diphyllobothrium, Hymenolepis, Echinococcus SUBCLASS : Cestodaria 1. A small group of cestodes that show some similarity to trematodes. 2. Unsegmented leaf-like body without a scolex and strobila(monozoic). 3. Body contains only one hermaphroditic reproductive system. 4. Trematode like suckers are sometimes present. 5. Digestive tract absent and the larvae resemble the larvae of tapeworms hence placed in class cestoda. 6. They are intestinal and coelomic parasites of sharks, rays, and primitive bony fishes. Example: Gyrocotyle that lives in fish. BIBLIOGRAPHY 1. Adaptation: Fitness of an organism for its environment. 2. Apolysis: Shedding of gravid proglottids in taenia. 3. Axial gradient: The capacity of regeneration along the antero- posterior axis due to difference in intensity of metabolic activity. 4. Bulbous pharynx: The pharynx is characterized by a sucking muscular bulb as found in platyhelminthes.

5. Cirrus sac: A pouch containing seminal vesicle, prostrate gland and cirrus of some platyhelminthes. 6. Coracidium: A ciliated free swimming larva of Diphyllobothrium latum (a cestode). 7. Definitive host: A host which harbours the adult stage of the parasite and in whose body the sexual mode of reproduction takes place. 8. Direct development: Without the occurrence of any larval stage. 9. Ectoparasite: A parasite that lives outside on the surface of the body of the host. 10. Endoparasite: A parasite living within the hosts body. 11. Gynaecophoric canal: The infolding of the lateral margins of the body of male behind the ventral sucker for holding the female in Schistosoma 12. Host: An organism which harbours the parasite. 13. Intermediate host: A host that harbours the larval stages of the parasite and in whose body the development of larva takes place. 14. Laurers canal: Represents the rudimentary vagina in platyhelminthes during the breeding season. 15. Parasite: Organisms that live on the expense of other animals for food, shelter, and dispersion. 16. Plerocercoid: The final stage in the life-cycle of Diphyllobothrium latum (a cestode). 17. Polyembryony: A developmental phenomenon in which the initial mass of embryonic cells give rise to more than one embryo.

Suggested Reading : 1. Invertebrate Zoology , by Robert D. Barnes Publisher: Saunders College International Edition (5th Edition) 2. Parker And HaswellText Book Of Zoology, Invertebrates, Volume 1 Edited by Marshall And Williams (7th Edition) A.I.T.B.S. Publishers And Distributors 3. Parasitology, by K. D. Chatterjee Publisher: 6, Amrita Banerjee Road, Kalighat, Calcutta 4. Biology Of Animals, by Ganguly, Sinha And Adhikari Publisher: New Central Book Agency 5. Invertebrate Zoology, by Ruppert/ Barnes Publisher: Harcourt Asia PTE Ltd. 6. Modern text book of Zoology, Invertebrates by R. L. Kotpal Publisher : Rastogi Publications 7. Text Book of Medical Parasitology, by C. K. Jayaram Paniker Publishers : Jaypee brothers