310 PLANARIA. ENTOZOA PROPER. 



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 aqua- 

 tic animals which it attacks ; mastering even the most active 

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

 them. The most curious parts 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. 



922. Although, for the sake of brevity, we have retained the 

 class of Entozoa as a single section of the Articulate series, and 

 the animals included in it agree in at least one important anato- 

 mical character, namely, the absence of transversely striated 

 muscles, it must be admitted that they present very great differ- 

 ences both in form and structure, and the majority of modern 

 naturalists are agreed in regarding them as forming two inde- 

 pendent classes, which have been denominated by Vogt, NEMA- 

 TELMIA or Round-worms, and PLATYELMIA or Flat-worms. 

 In the NEMATELMIA, which present the nearest approach in 

 form to the lower members of the preceding class, the body is 

 elongated and cylindrical, and the skin often exhibits indications 

 of segmentation in the form of slight wrinkles. There is usually 

 a distinct intestinal tube with an orifice at each end, in some 

 species a vascular system has been detected, and in most there 



