502 ZOOLOGY. 



covered in these animalcules, and even a ganglionary nervous 

 system. 



587. The rotifera wheel bearing animals (Fig. 177), 

 one species of which has become celebrated by the experiments 

 of Spallanzani on the suspension of life which follows its drying 

 up, may be taken as a type of the class. Their body is elon- 

 gated, and is terminated anteriorly by two small coronse of 

 cilia, which at the will of the animal are withdrawn into the 

 interior or expanded externally, and which by their vibrations 

 produce the image of two small wheels turning rapidly on 

 their axes. They terminate in a bifurcated and articulated 

 tail, by which they attach themselves to bodies on which they 

 wish to rest ; finally, two small red points seem to represent 

 the eyes. These animalcules swim with the greatest vivacity, 

 and lay oval eggs. 



588. Other animalcules, called branchions, resemble the 

 rotifera in the general mode of their organization, but merit 

 notice by reason of a sort of carapace, or shell, with which 

 their body is covered. In several of these small beings the 

 shell or covering is even bivalve, and recals very much that 

 of certain Crustacea, such as the cypris and daphnia. 



CLASS OF TUEBELLAEIA. 



589. This class ought to comprise a certain number of 

 vermes, whose body, more or less compressed, presents 

 scarcely any traces of annulation, and is covered with ex- 

 tremely fine vibratile cilia. In general they have no anus, 

 and their digestive apparatus is ramified, and terminates in a 

 cul-de-sac ; their nervous system is composed of two lateral 

 cords, terminating anteriorly in a pair of cerebroid gan- 

 glions, and they have distinctly-formed bloodvessels. Some, 

 as the nemertes and the planaria, live in water. Others, as 

 the fluke (Fasciola Hepatica, Fig. 530), are parasites, and 

 live in the interior of other animals. 



CLASS OF HELMINTHES, OE NEMATOlDS. 



590. This division is composed of a part of those animals 

 sometimes called intestinal worms, by reason of their living 

 generally as parasites in the intestinal canal of man and of 



