406 GENERAL ZOOLOGY 



PHYLUM 7— NEMATHELMINTHES 



Nemathelminthes (threadworms or roundworms) are non- 

 ciliated, unsegmented worms circular in cross section, elongated 

 and threadlike in form; some are free-living in water or moist 

 earth and others parasitic in animals and plants. They lack 

 paired appendages, though bristles, hairs, and suckers may be 

 present. Most are dioecious. 



CLASS I. NEMATODA. A smooth firm cuticula, overlying a 

 softer subcuticula, forms the body covering. The mouth is 

 at the dorsal side. The alimentary canal is a slightly differ- 

 entiated tube lying loosely in a body cavity and provided with 

 an anal opening near the posterior end of the body. The 

 body cavity is a hemocoel and lacks a peritoneum. The sub- 

 cuticula on each side projects into the body cavity, forming two 

 longitudinal ridges (lateral lines) each of which contains 

 an excretory tubule connected with the body cavity by a 

 ciliated funnel or nephrostome. The two excretory tubules 

 open to the outside by a single pore on the ventral side behind 

 the mouth. A dorsal and ventral band of the form of longi- 

 tudinal muscle fibers bulges into the body cavity. Each band 

 of muscle is divided into right and left portions by a longi- 

 tudinal nerve trunk. There are no circular muscle fibers. 

 The nervous system consists of a ganglionated ring encircling 

 the esophagus, and of a number of longitudinal nerves. Simple 

 eyes and sensory papillae are present in some. Most nematodes 

 are parasitic and dioecious. A few are viviparous. They 

 are very widespread and the number of species has been esti- 

 mated at over 100,000. 



Examples: Trichinella spiralis, the cause of trichinosis, lives 

 as an adult (male 1.5 mm. in length, female 3 to 4 mm.) in the 

 small intestine of man, the pig, rat, dog, and mouse, where they 

 reproduce sexually. The female after copulation penetrates 

 the mucosa and gives birth to from 1,500 to 10,000 young. 

 The latter migrate via the blood and lymph to the muscles of the 

 thorax, neck, and jaw, in which they become encysted, thereby 

 injuring the muscles to such an extent that death of the host 

 may ensue. Man is infected by eating undercooked pork con- 

 taining cysts from which the young worms are liberated by the 

 action of the gastric juice (Fig. 234). 



