Topic - ASCARIS LUMBRICOIDES

B.Sc 1st year Zoology Hons. Paper - I By Dr. Medhavi Sudarshan (Assist. Prof.) Zoology Department JNL College ,Khagaul Classification • Habitat: Ascarislumbricoides, common saying “round worm of man”, is the largest of the Kingdom:Animalia intestinal nematodes parasitizing humans. It is the Phylum:Nematoda most common worm found in human. It is Class:Chromadorea worldwide in distribution and most prevalent throughout the tropics, sub-tropics and more Order:Ascaridida prevalent in the countryside than in the city. Family:Ascarididae • Shape: Ascaris is a cylindrical worm, tapering at Genus:Ascaris both ends. Posterior end of male is curved. Species:A. lumbricoides • Colour: white to light pinkish in colour • Size: Female measures 30-40 cm and male 15-30 cm. Morphology: 1. Four longitudinal streaks run through the body. 2. The triangular mouth is surrounded by one mid-dorsal and two ventrolateral lips. 3. The dorsal lip has two double sensory papillae and ventrolateral lip carries one double sensory papilla each. The ventrolateral lips also have a lateral papilla each and a chemoreceptor called amphid. 4. There is a pair of cervical papillae and a pair of cephalic papillae on the dorsal side of the head. 5. Tail is straight in female and carries a transverse anus and a chemoreceptor called amphid between the anus and the tip of tail. In male the tail is curved and anus is replaced by cloaca from which two equal chitinous spicules or penial setae project out. In male two rows of reanal papillae and single and double post anal papillae are present. 6. Epidermis is the outermost syncytial layer covered on the outer surface by cuticle, which is a thick, transparent and noncellular layer secreted by the underlying epidermis. Epidermis is also called hypodermis as it lies below the thick layer of cuticle. 7. Below the skin muscles form a single layer of spindle-shaped cells that line the body cavity. The muscle layer is divided into four longitudinal columns, two doors-lateral and two ventro-lateral. Circular muscles are altogether absent. 8. Pseudocoelom is characteristic body cavity of round worms. It is filled with protein rich pseudocoelomic fluid and carries five giant mesenchyme cells, known as pseudocoelomocytes that occupy fixed positions in the anterior third of the body. Pseudocoelomic fluid is composed of about 93% water and substances such as protein, glucose, non-protein nitrogenous substances, sodium chloride and phosphate. 9. Digestive System Alimentary canal is a straight tube extending from mouth to anus and comprises mouth, a short pharynx or esophagus, a long tubular intestine and a short rectum. Mouth is triradiate leading to pharynx or esophagus that opens posteriorly into a thin walleddorsoventrally flattened intestine or midgut, which extends to the entire length of body. The posterior part of intestine opens into rectum that terminates in anus or cloaca. Food consists of blood which is sucked by the pumping action of pharynx. Digestion is extracellular in intestine. Digested food is absorbed by the intestinal cells and distributed by the pseudocoelomic fluid. Some intestinal cells also engulf small particles by phagocytosis and digest them intracellularly. Food digested by the host intestine is also directly absorbed through the skin. 10. Respiratory system Ascaris respires anaerobically because of lack of oxygen in host intestine. In this process, glycogen undergoes glycolysis and finally yields carbon dioxide, fatty acids and energy. Aerobic respiration also seems occur if free oxygen is available in host intestine, as small amount of cytochrome is present in round worms 11. Excretory System There is a single giant H-shaped rennet cell forming excretory system. It consists of two lateral longitudinal excretory canals, connected below the pharynx by transverse canalicular network. A short terminal excretory duct extends from the left side of canalicular network to the excretory pore situated midventrally on the anterior side of body. Nitrogenous wastes, chiefly urea, are collected from different parts of body. Pressure of pseudocoelomic fluid helps in ultrafiltration. 12. Nervous System The nervous system consists of circumpharyngeal ring that carries a dorsal ganglion, a subdorsal ganglion, lateral ganglia and ventral ganglion. From the circumpharyngeal ring six small nerves supply the sense organs of the anterior side. Posteriorly six longitudinal nerves arise from the ring and reach the posterior end. They are mid dorsal nerve, mid ventral nerve, one pair of dorsolateral nerves and one pair of ventrolateral nerves. 13. The dorsal and ventral nerves are connected by a number of transverse commissures. 14. The sense organs are as follows: Labial papillae are gustoreceptors and are present on three lips. Cervical papillae are tactile organs. Anal papillae are situated below the posterior end of male. There are 5 pairs of post anal genital papillae and 50 pairs of preanal papillae. They are tactile in function and help in copulation. Amphids, present on each lateroventral lip, are chemo receptors. Phasmids are chemo receptors that open one on either side of tail of female. 15. Reproductive system Sexes are separate, readily distinguishable by the tail and cloaca. (a)Male reproductive organs are confined to the posterior part of the body and consist of testis, vas deferens, seminal vesicle, ejaculatory duct and penial setae. The testis is a long, thread-like, coiled tube that continues distally into a short and thick vas deferens. The vas deferens joins posteriorly with a straight tube called seminal vesicle, which narrows at its posterior end to form a short but muscular ejaculatory duct which opens into the cloaca. Dorsal to the cloaca is a pair of penial sacs that contain a pair of spicules or penial setae, which serve in copulation. (b) Female reproductive organs consist of ovaries, oviducts, uteri and vagina. Ovaries are long, thread-like and highly twisted and lead to broader oviducts. Each oviduct continues into still broader and muscular uterus. The first part of uterus serves as seminal receptacle where sperms are stored and fertilization occurs. The remaining part of uterus stores fertilized eggs and its glands produce yolk and material for egg shells. The two uteri unite and open into a short median, muscular vagina that opens by transverse gonophores to the outside. Structure LIFE CYCLE

1. Ascaris is monogenetic since it has only one host, man in which infection occurs directly. 2. Cleavage is spiral, determinate and holoblastic. The first larva is formed 10-14 days after cleavage and is called rhabditiform larva or first larva which is not an infective stage. In another week, it moults to become the second larva, which is capable of infecting the host. 3. Man acquires infection by directly ingesting Ascaris eggs along with contaminated food or water. The second larva hatches out of the egg in small intestine and penetrates the intestinal wall to enter blood stream. Through the hepatic portal circulation it is carried to liver and then to the heart and is finally transported to lungs via pulmonary artery. 4. Within the alveoli of lungs, the second larva moults to become the third larva and grows in size. After about a week, it moults to become fourth larva that reaches pharynx through trachea. From pharynx it is coughed up and swallowed to reach the intestine, where it moults to become an adult.

PARASITIC ADAPTATIONS

Ascaris exhibits the following anatomical and physiological adaptations to live as endoparasite in the intestine of man. 1. body is long, flexible and cylindrical in shape with both the ends pointed to adjust to the narrow lumen of intestine. Body is covered externally by cuticle that helps to resist digestive juices of the host. 2. The worm also secretes antienzymes to protect it from the digestive enzymes of the host. 3. Adhesive organs are not found but there are papillae on lips to anchor on to the villi of intestine. Muscular pharynx facilitates ingestion of tissue and blood by sucking action. 4. There is a continuous supply of food digested by the host intestine, which it absorbs readily through its skin. There are no elaborate digestive glands as it feeds on partially digested food. Pseudocoelomic fluid serves carries out absorption, transport and distribution of food, oxygen and wastes. 5. Mode of respiration is obligatory anaerobic as it lives in an oxygen deficient environment in intestine. 6. Millions of eggs are produced which compensates for the death of large number of themin the course of transfer to the host. The hard shell of eggs protects the developing juveniles from the environmental hazards. 7. Direct infection to man through contamination and abundance of eggs in the environment increases its chances of reinfection of man.