Ascaris

Reproductive system

Sexes are separate, readily distinguishable by the tail and cloaca.

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.

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 a transverse gonopore to the outside.

LIFE CYCLE

Ascaris is monogenetic since it has only one host, man in which infection occurs directly.

Cleavage is spiral, determinate and holoblastic. The firstlarva is formed 10-14 days after cleavage and is called rhabditiform larva or firstlarva which is not an infective stage. In another week, it moults to become the secondlarva, which is capable of infecting the host.

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.

Within the alveoli of lungs, the secondlarva moults to become the thirdlarva and grows in size. After about a week, it moults to become fourthlarva 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.

• 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.
• The worm also secretes antienzymes to protect it from the digestive enzymes of the host.
• 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.
• 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.
• Mode of respiration is obligatory anaerobic as it lives in an oxygen deficient environment in intestine.
• Sense organs are very poorly developed because of endoparasitic life.

Millions of eggs are produced which compensates for the death of large number of them in the course of transfer to the host.

• The hard shell of eggs protects the developing juveniles from the environmental hazards.
• Direct infection to man through contamination and abundance of eggs in the environment increases its chances of reinfection of man.