84 ZOOLOGY. 



118. Nematoidea (Gr. nema, thread; eidos, form). To 

 this group belong the round-worms, thread-worms, 

 vinegar eels, &c. Most of them infest the intestines of 

 warm blooded quadrupeds. The round-worm sometimes 

 attains a length of fifteen inches in the human intestine. 

 The thread -worm inhabits the large intestine. It is found 

 principally in children. The Trichina passes its early 

 stage in the muscles of the pig, and is often transferred 

 to the human stomach where it is developed. Other 

 species infest the intestines of the horse and pig. These 

 animals possess a mouth and an alimentary canal with 

 anal opening. They have also a water- vascular system. 

 A ganglionated nervous cord surrounds the gullet, from 

 which nervous threads proceed towards the extremity of 

 the body. The sexes are distinct. 



There are many species of non-parasitic Nematode 

 worms found in fresh water or on the sea shore. The 

 vinegar eel is a well known specimen. 



119. Acanthocephala (Gr. acantkos, thorn; cephale, the 

 head). These are formidable internal parasites, which 

 inhabit the alimentary canal of many mammals, birds, and 

 fishes ; but have not yet been found in the human in- 

 testine. They are so called on account of the recurved 

 hooks with which their proboscis or snout is surrounded. 

 By means of these they attach themselves to the mucous 

 membrane of the intestine. There is a single nervous 

 ganglion at the base of this proboscis. There is a network 

 of canals beneath the skin, containing a clear fluid, which 

 is believed to be a modified form of the water-vascular 

 system. They have neither mouth nor alimentary canal. 

 The sexes are distinct. 



120. Gordiaceae (so called from the " Gordian Knot," be- 

 cause they twist their bodies into knots), or hair-worms, 

 are popularly believed to be living horse hairs. They pass 

 the early stages of their lives in the interior of insects, 

 especially grasshoppers and water-beetles. They attain 

 a length of ten or eleven inches, and then leave the body 

 of their victim to breed. They deposit theiv eggs in 



