108 



THE OiUGIN OF PARASITES. 



FIG. 69. Ciliated Embryos of A, Ditto- 

 mum hepaticum, and S, of Monostomum ca- 

 pitellatum ; the former with an eye-speck. 



tous body contains a many-branched alimentary canal without anus, 

 and with a powerful pharynx and a strongly developed hermaphrodite 

 sexual apparatus. The same agreement obtains in the structure and 



arrangement of the excretory vessels, 

 the nervous system, and the muscles. 

 Even in respect of the histology 

 there are many similar agreements. 

 Finally, since the embryonic condi- 

 tions also manifest great similarity 

 to each other, there remains a differ- 

 ence between the two groups, only 

 inasmuch as the one consists of free- 

 living animals, the other contains 

 only parasites. The specific peculi- 

 arities, however, of the Planarians, 

 as well as of the Trematodes may 

 be ascribed to this difference ; since 

 the possession of a ciliated epithe- 

 lium and special organs of sense, as 

 we find them in the Planarians, cor- 

 respond with the requirements of a 

 free life, exactly in the same way as 

 the presence of a hook-apparatus does to the conditions of parasitism. 

 The free swimming young forms of the Trematodes even their 

 entozootic species are mostly provided with the ciliated epithelium 

 of the Planarians, and often also with the eye-specks of their free- 

 living relatives (Fig. 69). 



There are forms, even in the fully developed condition, which 

 serve as connecting links between the two groups. As there are 

 numerous species of Trematodes which, instead of inhabiting the 

 internal organs, live upon the external surface of their host, 

 and approach free-living animals in their pigmentation and pos- 

 session of eyes, so also we are acquainted with Planarians, the 

 posterior extremity of whose body presents a discoid organ of 

 attachment (Monocelis caudatiis, Oulian.), or even bears a true 

 sucker (Monocelis protractilis, Greeff), by the help of which they 

 attach themselves to foreign bodies. Leidy erects the Planaridse, with 

 a suctorial disc at the posterior extremity of the body, into a distinct 

 genus (Bdellura), and describes in it a species (Bdellura parasitica) 

 which lives on the gills of Polyphemus occidentalis, and presents an 

 instance of a true parasite. * Apart from the ciliated epithelium, it 



1 Here may also be mentioned Malacobdclla, which was for a long time classed among 

 the Trematodes, and, like the Entozoa, is parasitic in shell-fish, but notwithstanding belongs 



