314 PLANARIA. ENTOZOA PROPER. 



851. There is a very interesting group, consisting of the 

 genus Planaria and its allies ; which is placed in this class on 

 account of its general conformity with it in structure ; although 

 not agreeing as to the residence of the animals composing it, 

 these not being inhabitants of the bodies of other animals, 

 but swimming freely in waters of the ocean, as well as in 

 streams and ponds, and crawling upon their banks, or upon 

 floating substances. The body is flat, and three or four times 

 as long as it is broad. Within its soft tissue are channelled 

 out, not only a complex digestive cavity, but also a system of 

 vessels which absorb fluid from its walls, and convey it through 

 the system. The stomach opens, not by a mouth at one end, 

 but by a sort of sucker projecting from the middle of the body; 

 and through this the Planaria imbibes the juices of various 

 aquatic animals which it attacks ; mastering even the most active 

 little worms (such as the Nais) by twisting its body round 

 them. The most curious part of the economy of these animals 

 consists in their power of reproducing parts that have been lost, 

 and of repairing injuries ; which seems to be almost as great as 

 that of the Hydra among Polypes. They may be divided into 

 three parts ; of which the first shall contain the two minute spots 

 which are believed to be eyes ; the middle one the sucker ; and 

 the posterior one the reproductive apparatus; and in a short 

 time, each part will develope itself into a new individual, perfect 

 in all its parts. It may be partially split longitudinally from 

 either extremity, so that two heads, or two tails, or both in com- 

 bination, may be formed, all uniting at the middle point, but 

 each being complete in itself. 



852. The proper ENTOZOA do not present many points of 

 general interest ; although their peculiar relations to Man, and 

 the numerous varieties of structure which they present, render 

 them objects of much scientific interest to the professed Naturalist 

 and Physiologist. For obvious reasons we shall here content 

 ourselves with a very general sketch of the group. It may be 

 divided into two primary Sections, according to the degree of 

 development of the principal organs. In the first and highest 

 of these, the STERELMINTHA, there is a distinct intestinal tube 



