Evolution of Animals 387 



inhabit fresh water, whilst some are brackish and a few marine ; 

 84 genera and about 700 species have been described." Of 

 these and more recently discovered types some 73 genera are 

 fresh- water and 11 or 12 are brackish- or marine. The embry- 

 ology and the mature structure both indicate that the group 

 probably sprang from a common ancestry with the turbel- 

 larian and nemertean worms as well as with the primitive 

 brachiopods and molluscs. For all possess in common a more 

 or less extensively ciliated trochosphere larva in which the 

 alimentary, the nervous and sense organs, the circumoral 

 disk, and other structural details agree (p. 507). 



But, while nearly all of the above-named higher groups 

 have greatly advanced in structural complexity and in size, 

 the Rotifera have remained comparatively retarded and may 

 be said to represent permanently the post-trochosphere or 

 subveliger stage of turbellarians, molluscs, and other groups. 



So far as the writer is aware a marine origin — even a littoral 

 marine origin — has never been claimed for them. The fact 

 also that about 20 living species have a marked capacity of 

 adaptibility to fresh-water, to brackish, or to marine life sug- 

 gests that the marine types have slowly become adapted from 

 an originally fresh-water environment. The small size and 

 soft nature of the animals explain their absence from geologic 

 records. 



The large and world-wide group of the flatworms or Platy- 

 elminthes presents many interesting problems in structure 

 and distribution. The evident transitions shoAvn from the 

 free-swimming turbellarians to the usually parasitic trenia- 

 todes, and the highly modified parasitic habits of the cestodes, 

 all involve suggestive questions of phylogeny that cannot 

 now be considered. No one, we believe, mil question the 

 land origin of at least two of these. 



Numerically the polyclad turbellarians include 28 to 29 

 genera, all of which are marine; the triclad division consists 

 of 27 genera that are either fresh-water or terrestrial and 8 

 that are marine; while the rhabdocctle division consists of 

 38 genera that are marine and 24 that are fresh-water. The 



