182 



TEXTBOOK OF ZOOLOGY 



of the intestinal wall. Ordinarily the coelom, when fully developed, 

 is lined both laterally and medially with mesodermic peritoneum. 

 This is the simplest type of animal in which the body cavity or 

 coelom is found. In higher forms the outer coat of the intestine is 

 mesodermic. The alimentary canal is quite straight and simple and 

 lies in the dorsal part of the body cavity. There is no need for 

 great specialization of the digestive system since the food is taken 

 from the digested material in the intestine of the host. A contrac- 

 tile pharynx, which acts as a pump, draws fluid into the long 

 epithelial intestine from which it is absorbed by the other tissues. 

 The narrowed posterior portion is the rectum and leads to the anus 



fertilization 

 /Tjemi^ranC' 



Sfye// 



ze/^ade 



Fig. 92. — Fertilized ovum, 4., and amoeboid spermatozoa, B, of Ascaris lumbri- 

 coides. (From Curtis and Guthrie, Textbook of General Zoology, published by- 

 John Wiley & Sons, Inc., after Leuckart. ) 



at the posterior portion of the body. The two laterally located, 

 longitudinal duets open externally by a single pore near the anterior 

 end of the body. There is a nerve ring around the pharynx which 

 gives off a large dorsal longitudinal nerve and a large ventral 

 longitudinal nerve. There are usually four other smaller longitudi- 

 nal nerves and some connectives. In the males the testis is a thread- 

 like structure which is much coiled in the cavity. This tube enlarges 

 posteriorly to become the vas deferens which in turn enlarges still 

 more before reaching the aperture to become the ejaculatory duct. 

 In the female the threadlike ovaries join the coiled oviducts which 

 lead forward and join the two uteri. These tubes join in the vagina, 

 which is a short tube leading to the genital pore. 



