388 Causes and Course of Organic Evolution 



total of the marine genera, therefore, is 75, and of the hind or 

 fresh-water 51. But thanks to the detailed treatment in Von 

 Graff's splendid work (131) we learn that, of the simplest, 

 and as we believe originally lacustrine, group Rhabdocoela 

 proper, 167 species are fresh-water, and 153 are marine. 



A point worthy of at least passing notice is that in the other- 

 wise marine genus Plagiostoma the species P. lemani is found 

 in Lake Geneva. 



Of the above groups then the rhabdocoel turbellarians are 

 evidently most primitive and tj^pical in structure, while the 

 sub-group Rhabdocoela, that includes fresh-water, terrestrial, 

 and littoral marine species, may well have been the originating 

 one for other and more modified types, some of which like 

 the triclad and polyclad groups have greatly evolved, while 

 others like the Acoela have become degenerate types. 



The Trematoda is made up of 45 genera of fresh-water or 

 land parasites and 10 that are marine, but, when one finds 

 genera like Distomum and Gasterostomum, that parasitize on 

 land or on fresh-water animals, to include one or two species 

 in each genus that inhabit marine fishes, the question may 

 well be asked whether the entire group has not been of land 

 origin. 



In the tliird group the members of the Cestodes or tape- 

 wonns are so evidently adapted in the great majority of cases 

 to a parasitic life in land or lacustrine animals that we would 

 regard the few marine ones as derivative and adapted types. 

 Sixty to sixty-five genera may be taken as making up the group. 



Van Beneden's interesting though restricted group of the 

 Mesozoa is so far as known wholly marine and parasitic, l^iit 

 it may well be that they represent very ancient forms remotely 

 descended from simple land ancestors allied to the Turbel- 

 laria and Trematoda, and which have remained alive through 

 assuming the parasitic habit. 



The small group Gastrotricha includes about 50 species 

 that are fresh- water in ha])it. 



The present-day forms of the Nemertinea are largely marine, 

 but it is highly significant that two genera of the more com- 



