184 GENERAL ZOOLOGY 



of odd situations. Many may become parasites if there is 

 opportunity to do so, and a large number of species are 

 permanently parasitic. As an example of the group a 

 parasite of man will be considered. 



Ascaris Lumbricoides Linnaeus 



Self -maintenance. Ascaris is a round-worm nearly a foot 

 long which lives in the small intestine of man. It obtains 

 its nourishment from the partly digested food of its host. 

 As long as the man eats, the worm has a plentiful supply. 

 There are three lobes at the anterior end which act as lips 

 and poke food into the mouth. These are assisted by a 

 muscular pharynx which exerts a sucking action and draws 

 particles inside the body. Behind the pharynx the food 

 traverses a straight intestine, where it is digested and 

 absorbed. Undigested fragments pass out through the 

 anus near the posterior end of the body. No special cir- 

 culatory system is present, but the absorbed food passes 

 through the wall of the intestine into the body-cavity 

 (Fig. 76, B), where it is free to move from one end of the 

 body to the other, and all parts are thus supplied. Excre- 

 tions are collected by two tubes which run longitudinally on 

 either side of the body (ex) and open together to the exterior 

 through a small pore on the ventral side near the anterior 

 end. 



Self -protection. Ascaris is in no danger from predatory 

 enemies, for it rests safe inside some man. It is not digested 

 by the strong secretions in the intestine, as the cuticle on 

 the outside of its body is impervious to most chemicals 

 and furnishes adequate protection. If the man containing 

 an ascarid dies, the worm leaves the intestine and is often 

 found, on post-mortem examination, in the stomach, throat 

 or mouth. If the worm succeeds in escaping from the body, 

 it is able to live for some time and may perhaps find en- 

 trance into another host. As a rule, however, Ascaris is in 

 no danger if it once finds a host but is able to pass its life 

 in quiet and seclusion. 



