THE PHYLA ASCHELMINTHES AND NEMERTEA 229 



many of its general anatomic features are visible. The pharynx ends in 

 a posterior enlargement, the bulb, which leads directly into a long sim- 

 ple intestine. The intestine ends in a short rectum that opens at a 

 postero-ventral anus. The vinegar eel, like many free-living nematodes, 

 feeds primarily on bacteria. 



A nerve ring is the only visible part of the nervous system; it sur- 

 rounds the pharynx just in front of the bulb. 



The sexes are separate. Males have a single thin testis that passes 

 forward from just in front of the anus, then doubles back on itself and 

 continues as a sperm duct to a storage expansion, the seminal vesicle, 

 that opens into the rectum. Instead of a penis the male has a pair of 

 copulatory spines mounted in the dorsal wall of the rectum. These can 

 be protruded through the anus and into the vagina of the female by the 

 contraction of protractor muscles. 



A single ovary lies in the middle third of the female (Fig. 12.6). 

 From its anterior end an oviduct, widened to form a uterus, leads back 

 to the vagina, just posterior to the middle of the body. The vagina 

 opens ventrally. Posterior to the uterus a diverticulum serves as a 

 seminal receptacle for receiving sperm at copulation. Eggs produced in 

 the ovary are fertilized as they pass into the uterus by sperm that mi- 

 grate forward from the receptacle. Eggs are retained in the uterus until 

 they hatch, and the young worms escape through the vagina. Thus, the 

 vinegar eel is ovoviviparous. 



90. The Pig Roundworm, Ascaris Lumbricoides 



Further details of nematode anatomy are more easily seen in the 

 few large species, all of which are parasitic. Ascaris lumbricoides is a 

 foot long, and may be obtained from pig intestines at slaughterhouses. 

 This species differs from the free-living species primarily in having more 

 prominent reproductive organs. The mouth and pharynx are somewhat 

 reduced. 



As Ascaris is cut open, the large pseudocoelom (Fig. 12.7) is evi- 

 dent. In it the long intestine and much-folded reproductive organs lie 

 loosely. On the wall are lateral, dorsal and ventral lines, and the inner 

 surface is covered with small transparent sacs. These represent some of 

 the more bizarre cell structures in nematodes. 



The lateral lines are internal ridges, each containing an excretory 

 canal that runs the length of the worm. The two canals join beneath 

 the pharynx and a short common tube runs forward to open just be- 

 hind the mouth as an excretory pore. The entire excretory system, often 

 a foot long, is made from a single cell, whose nucleus is located where 

 the two tubes join together. At their inner ends the tubes are closed. 

 Flame cells are lacking, and little is known of the physiology of this 

 system. 



The dorsal and ventral lines are the nerve cords that extend back 

 from the nerve ring around the pharynx. The brain in most nematodes 

 is located in the swollen sides of this ring, connected above and below 



