Flatworms and Threadworms 301 



and expansion it can draw the host's food into itself. At the posterior 

 end of the digestive tube the intestine becomes smaller. This is the 

 rectum, which empties through the anal opening. 



The Excretory System. 



This system consists of two longitudinal canals, one located in each 

 lateral line. These open through a single pore near the anterior end of 

 the ventral body-wall. 



The Nervous System. 



A definite ring of nervous tissue surrounds the pharynx. From this 

 ring a dorsal and a ventral nerve cord are given off, as well as a number 

 of fine nerve strands and connections. 



The Reproductive System. 



In the male there is but a single testis, which is coiled and thread- 

 like. The sperm cells pass from this through a vas deferens to a seminal 

 vesicle and from here through the ejaculatory duct to the rectum. 



In the female the reproductive system is Y shaped, the two arms 

 of the Y being the coiled ovaries which are continuous with the uterus. 

 It is the two uteri which unite in the stem of the Y to form a muscular 

 tube, the vagina, which opens to the outside of the body by a genital 

 aperture. 



The egg is fertilized in the uterus, after which a chitinous shell 

 surrounds it, and the egg is then thrown out through the genital pore. It 

 is this chitinous shell which prevents the egg from being digested in 

 the intestine of the host where it must necessarily fall when laid. 



As nematodes are triploblastic animals with three definite germ 

 layers, these animals also have a coelom. Consequently, the body of 

 these worms must be thought of as a tube within a tube, with the repro- 

 ductive system lying between the digestive tract and the body wall — 

 that is, within the coelom. 



However, the coelom is quite different in worms from what it is 

 in higher animals. In the higher forms, the coelom is a cavity between 

 the two layers of mesoderm. The excretory organs open into it and 

 from its walls the reproductive cells originate. In Ascaris the coelom 

 has only the mesoderm of the body wall as a lining. There is no meso- 

 dermal lining surrounding the intestines. Then, too, the excretory or- 

 gans open directly to the outside through the excretory pore, and the 

 reproductive cells do not originate from the epithelium of the coelom. 

 Notwithstanding this difference, the space between the intestinal tract 

 and the body wall is called coelom in worms. 

 Nematode Infections (Figs. 185, 186, 187, 188). 



Ascaris lumbricoides is found chiefly in children. The female is 

 from seven to twelve inches in length, and the male from four to eight 

 inches. The worm is pointed at both ends and of a yellowish-brown or 



