230 



THE ANIMAL KINGDOM 



Cuticle' 

 G astro derm 



Testis 

 Sperm duct 



VeiT-tral nerve 



Dorsal nerve 



Cuticle 



Epidarmis 



Pseudocoelom 



Lateral line 



Excretory 

 canal 



Cell tody of 

 muscle cell 



Conti^actile portion 



Conductile prooess 



Figure 12.7. Cross section through the inicklle region of a male A.sraiis lunibricoides. 

 The testes are sectioned several times because they lie folded in the body. 



the pharynx by many nerve fibers. In Ascaris the brain is scattered out as 

 several pairs of ganglia associated with the ring. 



The small transparent sacs lining the body wall, easily visible to the 

 naked eye, are the cell bodies ol the muscle fibers. Each fiber extends 

 longitudinally one quarter to one halt inch beneath the epidermis. At 

 its middle is the sac hanging into the pseudocoeloin. The cell body 

 contains the nucleus and is not contractile. The muscle cells of nema- 

 todes are not innervated by nerve fibers coming from the nerve cords as 

 in most animals. Instead each muscle cell sends a conductile process to 

 the nerve cord (Fig. 12.7). Thus each muscle cell has three portions, and 

 is structurally unique in the animal kingdom. 



The life cycle of Ascaris involves only a single host. The pig round- 

 worm may lay as many as 200,000 eggs per day. These pass out of the 

 pig in its feces, where the egg develops into a small worm within its 

 shell. If these contaminate the food of pigs and are eaten, they hatch 

 in the intestines. They then go through a seemingly unnecessary cycle. 

 The young worms burrow through the intestinal wall into the blood 

 stream, whence they are carried through the heart to the lungs. Here 

 they burrow into the air spaces, crawl up the trachea to the pharynx, 

 and are swallowed again. They finally mature in the intestines. During 

 the burrowing phase, if large numbers are involved, hemorrhage, infec- 

 tion or pneumonia may result. 



Ascaris hunbricoides is a species complex of a number of morpho- 

 logically indistinguishable strains that variously infect pigs, sheep, squir- 

 rels, apes and man. Each strain can temporarily infect other hosts, but 

 can mature and reproduce only in its own host. 



