ROUNDWORM (ASCARIS) 339 



lips have small papillae on their surfaces, two on the dorsal and 

 one on each of the ventral. The shape of the body is generally 

 cylindrical with tapering ends. The smooth surface is marked by 

 four longitudinal lines, two lateral, one dorsal, and one ventral. The 

 genital pore in the female is located on the ventral midline approxi- 

 mately one-third of the length of the body from the anterior extremity. 

 The anus is located near the posterior tip of the body, and in the male 

 the reproductive aperture and two penial setae or spicules are located 

 just within this opening. 



Internal Anatomy 



The body wall is composed of the thin, outer, smooth cuticle, the 

 epidermis, whose cells run together, and a thick layer of longitudinal 

 muscle fibers, whose medial margins are rather baggy. There are 

 thickenings of the epidermis in the positions of the longitudinal 

 lines. The excretory tubes follow the lateral lines. The body cavity 

 of this animal is a primitive or false coelom which is lined externally 

 by the mesoderm of the body wall and internally by the endoderm 

 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 

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

 longitudinal ducts 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. 



