268 THE ANIMAL PARASITES OF MAN 



finally originate from the pharyngeal ring ; they consist each of 

 three fibres, carry three ganglion cells at their point of origin, 

 and enter the sensory organs of the three papillae surrounding the 

 oral aperture. Two of these little trunks lie in the lateral ridges, 

 the remaining four are situated in the middle of the four quad- 

 rants (Nn. submediani anteriores). 



The parasitical species lack higher ORGANS OF SENSE ; free- 

 living worms occasionally have two rust-red eyes, sometimes witli 

 lenses, at the front of the body. In addition to the above- 

 mentioned sensory papillae surrounding the oral aperture and the 

 genital papillae of the male at the end of the body, the Ascarides 

 possess still another pair, situated in the vicinity of the lateral 

 ganglia of the " cervical papillae " ; they, moreover, possess two 

 dorsal papillae in the central region of the body and two lateral 

 ones near the tip of the tail. 



THE EXCRETORY ORGANS of the Nematodes are variable. In a 

 great many cases the apparatus is symmetrical, and consists of 

 a tube commencing in the posterior extremity along each lateral 

 ridge (fig. 182), and passing towards the front. In the vicinity of 

 the anterior extremity both tubes pass out of the lateral ridges, 

 bend ventrally, and, in the median ventral ridge unite into a short 

 duct, which opens into an excretory pore (fig. 185, Exp.), and is 

 lined by a continuation of the cuticle of the body. Asymmetry is 

 occasioned through the excretory duct proceeding from the ventral 

 pore to the left at the lateral ridge, and then taking up the tube 

 in the anterior part, which is broader, and passes along the left 

 lateral ridge ; shortly before its union with the excretory duct it 

 throws out a branch to the right towards the lateral ridge, which, 

 however, always remains weak, and passes posteriorly into the 

 right lateral ridge ; a few smaller branches in addition spring from 

 the left principal part. In other species the right branch is 

 completely suppressed ; the entire organ thus lies along the left 

 lateral ridge, and consists of the excretory duct, which occasionally 

 opens quite in front near the lips, as well as the excretory canal, 

 which throws out a number of lateral branches. 



This excretory portion is one single elongated, or horse-shoe- 

 shaped cell, with a large nucleus and an intracellular tubular 

 system, which is connected with the excretory duct originating 

 from the outer surface. The so-called ventral gland is the only 

 excretory organ of marine Nematodes, and probably represents a 

 primitive form. 



In a number of Nematodes (C heir acanthus, Trichosomum, Tricho- 



