300 GENERAL BIOLOGY 



The Digestive System. 



The digestive system is very simple, consisting of a mere straight 

 tube into which the already digested food of the host enters. A definite 

 coelom may also be seen. The more anterior portion of the digestive 

 tube is known as the pharynx. This is muscular, so that by contraction 

 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 being 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 being digested in the in- 

 testine of the host where it must necessarily fall when being 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 re- 

 productive system lying between 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 meso- 



