130 INVERTEBRATE ZOOLOGY 



cate with the exterior through a single excretory pore on the 

 ventral surface of the body. In the dorsal and ventral lines are 

 found the two main longitudinal nerve trunks (Fig. 70 A) which 

 connect with a nerve ring near the anterior extremity of the body. 

 From this same ring are given off a number of smaller longi- 

 tudinal branches, some extending forward and others a short 

 distance backward. 



Digestive System. — The digestive system is practically a 

 straight tube (Fig. 70 B) with a mouth at the anterior extremity 

 and the anus on the ventral surface slightly in front of the 

 posterior tip. The mouth is usually surrounded by liplike organs 

 and, in the case of parasitic forms, frequently bears chitinized 

 structures for grappling the host tissues. Behind the mouth, 

 there frequently occurs a muscular esophagus with an especially 

 conspicuous pharyngeal bulb. The stomach-intestine continues 

 posteriorly from the esophagus as a usually flattened tube. 

 In the male, the genital ducts open into the posterior region 

 of the digestive tube which is thus transformed into a cloaca. 



Reproductive Organs. — The male is usually recognizable by its 

 smaller size and frequently by the more pointed and curved 

 posterior extremity. From the cloaca of the male, copulatory 

 spicules are frequently protruded. Reproductive organs in the 

 sexes are very simple in structure, for normally they consist of a 

 gonoduct which is directly continuous with the gonad. This tube 

 in the male is usually single, while in the female (Fig. 70 B) it is 

 frequently bifurcated with the opening of a single vagina occurring 

 on the ventral surface of the body. In some forms, the ducts and 

 gonads are extremely long and coiled throughout the greater 

 part of the length of the body, while in others they are a single 

 straight rod. 



The eggs of the horse Ascaris (A. megalocephala) have provided 

 one of the most widely used materials for the intimate study of 

 chromosomes and of the mechanics of mitosis and cleavage. The 

 works of Boveri, Biitschli, and zur Strassen on the eggs of Ascaris 

 stand as classics in the field of cytology. 



Method of Reproduction. — Nematodes reproduce only sexu- 

 ally. Eggs are fertilized within the oviduct of the female 

 and are discharged either before development has proceeded far 

 or are retained until the young are fully developed. In some 

 instances, the eggs are not laid but are retained in the body 

 of the female until the young have left the egg membranes and 



