44 ZOOLOGY. 



1. Trematoda. 



2. Planariidge. 



Subclass III. Cavitaria. 

 Having a visceral cavity, tlie nervous system bi-radiated. Composed of 

 four orders, the first having a visceral cavity instead of an intestine, the 

 remaining three with two openings at the alimentary canal, at opposite 

 extremities. 



a. Enterodela^ with one family : 



1. Acanthocephala. 

 h. Enterodela oylindrica [Nematoidea, Rudolphi]^ two families : 



1. Ascaridse. 



2. Gordiidae. 



c. Enterodela plicata [Acanthotheca, Diesing]^ a single family and 

 parasitic genus : 



1. Linguatulidae. 



d. Enterodela toenioida^ one family : 



1. jSTemertidae. 



The greater part of the Helminthes live in various parts of other animals, 

 deprived of light, with little occasion for locomotive powers, and governed 

 by circumstances of great uniformity, so that we need not be surprised at 

 the simplicity of structure in individual genera, although they present a 

 considerable amount of variation in the aggregate. From the mode of 

 their occurrence within other animals, they have been termed Entozoa^ in 

 which certain external species are included. In some cases the same 

 species is found in waters as well as in the interior of animals. There is 

 scarcely an animal, whether terrestrial or aquatic, which does not nourish 

 some of these parasites, and but few of the latter infest several distinct 

 species. They have been found in beasts, birds, reptiles, fishes, insects, 

 mollusca, and even in the acalephse. 



These entozoa are found, according to the species, in various parts of 

 the bodies they infest, as the intestines, brain, bronchise, liver, kidneys, 

 muscles, blood, and bones. 



I. Helminthophyta. 



a. Cystica. 



These are named from the resemblance of the posterior part of the body 

 to a bladder, a part which is filled with fluid {pi. 77, fig. 29). This is 

 fibrous and sensitive, at times as large as an egg, and it sometimes forms 

 part of several individuals. In Coenurus the head or heads are each 

 attached to a short neck ; they are sub-globular, crowned with hooks, and 

 have suckers arranged around the convexity, to enable it to attach itself 

 to the substance whence it derives its nourishment. It lives in the brain 

 and spinal nerve of sheep, and more rarely in the brain of oxen, destroying 

 parts of it, and giving rise to a fatal disease, called by the expressive name 

 of " staggers," from its chief symptom. 



In Echinococcus (called Acephalocyste by the French) there is no head 



* This name is inadmissible, having been previously used by Ehrenberg in the Infusoria, 

 p. 24. 



•:48 



