METAZOA VERMES. 321 



Although many naturalists, as we have said, place the 

 Platyhelminthes as primitive worms, yet at the same time 

 it is stated that even the Triclads belonging to the most 

 generalized division of this group, the Turbellaria, deposit 

 their ova in chitinous cocoons which contain, beside the 

 ova proper, large numbers of amoeboid cells originating 

 in the pouches of the parent and serving as food for the 

 young embryo. In association with this condition of 

 affairs many peculiarities of segmentation and growth 

 occur in the Triclad embryos all of which must be con- 

 sidered as secondary adaptations. 1 These facts being 

 true, it is evident that the Triclads cannot be primitive 

 worms. 



The Turbellaria include not only marine and fresh- 

 water forms but also many terrestrial species. 



TREMATODES. 



The Trematodes are built upon the same plan of struc- 

 ture as the Turbellaria but this plan is greatly modi- 

 fied by parasitism. All the group excepting the Temno- 

 cephala are either external or internal parasites, and there 

 are numerous adaptive characters and complicated modes 

 of development. A good illustration of the Trematodes 

 which pass through a metamorphosis is found in the 

 fluke worm, Distomum ( PI. 795 ; No. 796, a specimen 

 taken from the liver of the red deer). The eggs of most 

 flukes pass from the intestine of their host into water, 

 where, as in the case of Distomum hepaticum (PI. 795, 

 figs. 1-6) , they become ciliated larvae (fig. i). Very soon 

 this larva bores into the body cavity of a snail (Lim- 

 naeus minutus}. In this cavity it loses its cilia and be- 

 comes a sac-like body or sporocyst (fig. 2). Its germ 

 cells, seen in fig. 2, grow large and divide, giving rise to 



1 McMurrich, Invertebrate Morphology, 1894, p. 140. 



