226 NEMATHELMINTHES 



wriggling actively in fresh-water ponds and ditches, and look much like 

 thick horsehairs. Sometimes a number are found in a tangled mass, a 

 feature which suggested the name of the typical genus. As larvae the 

 worms live in the body cavity of insects, whence they migrate into the 

 water, their sudden appearance often giving rise to the common belief 

 that they are metamorphosed horsehairs. 



The integument consists of a thick cuticula and a hypodermis, the 

 latter being a single-layered epithelium and very different from the sub- 

 cuticula of nematodes. Beneath the integument is a muscle layer consist- 

 ing of a single layer of longitudinal muscle cells. The body cavity is 

 lined with a peritoneum and traversed by dorsoventral mesenteries and 

 is nearly filled with a mass of connective tissue cells forming a sort of 

 parenchyma. The mouth and oesophagus in adults are closed and the 

 intestine is a straight tube proceeding to the anus at the hinder end of 

 the body. Special respiratory, circulatory, and excretory organs are 

 absent. 



The nervous system consists of a nerve ring round the oasophagus 

 with two dorsal swellings and a median ventral cord. The sense organs 

 are a pair of eyes and numerous tactile bristles. The 

 sexes are separate; two testes and two ovaries are 

 present and in both sexes the reproductive organs open 

 to the outside through the anus. 



The eggs are laid in long strings in the water, the 

 length of one observed by Leidy being 91 inches, and 

 containing 6 million eggs. The young larvae (Fig. 

 360), after hatching, seeks some aquatic insect larva 

 into which it bores its way by means of bristles on the head. It remains 

 here in the muscles or fat body until the insect is eaten by some other 

 water insect or fish or has completed its larval life 'and left the water as 

 an adult. If in the latter case the host is eaten by a predaceous beetle 

 the larval worm may pass into its second larval stage in its body cavity, 

 or in !a grasshopper or other insect if the first host dies and the young 

 larva falls upon the ground. In its second host the worm grows rapidly 

 and assumes the long hair-like form of the adult, and finally breaks its 

 way through the body wall of its host and falls into the water or is swept 

 there by the rain, where it becomes mature. 



The class contains 2 families and about 15 American species. The 

 second of these families is very different from the first and its relationships 

 are rather obscure. 



Key to the families of Gordiacea: 



i Fresh-water and terrestrial worms 1. GOBDIIDAE 



Oj Marine worms 2. NECTONEMATIDAE 



