Applied Zoology
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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: ranjanasaxena1957@gmail.com 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). 2 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). 3 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 4 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 5 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 6 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.