Evolution of Animals 3S9 



plex types are not only land or fresh-water, they exhibit a 

 geographical distribution that is practically world-wide. Thus 

 the genus Sticliostemma includes seven species that are found 

 in Europe, Asia, North America, and East Africa; while Geo- 

 nemertes consists of eight species that are met ^^^th in New 

 Guinea, Australia, New Zealand, the Mascarenes, and Ber- 

 muda. 



As stated later we would regard the group as one of excep- 

 tional phylogenetic value (p. 418). It becomes therefore a 

 matter of some importance to determine whether the terres- 

 trial or swamp species have been derived from a marine ancestry 

 or vice versa. According to Blirger's beautiful monograph 

 (132)y supplemented by a few recent papers, the total number 

 of species is about 450. Of these 15 species are terrestrial 

 or fresh- water, while the remainder are more or less marine. 

 But when we find the genus Stichostemma widely distributed 

 over fresh-water areas, and Geonemertes to inhabit land areas 

 at varying elevation, even though the larger number is now 

 marine, there seems no inherent reason why the group should 

 not have had a ten-estrial or lacustrine origin, as has in part 

 been suggested by Montgomery. 



The large and diversified division of the polychaete worms 

 may truly be called marine, for of the 145 genera, that include 

 a much larger number of species, all are littoral and marine 

 (or pelagic) except two genera that may well have been passed 

 from the sea into a fluviatile life, while the genus Nereis that 

 is largely marine includes two fresh- water species. Though 

 most of them are soft-bodied, and have therefore left few 

 traces of their remains fossilized, the genus Spirorbis with its 

 calcareous test has been traced back into the silurian rocks, 

 so that the group is an ancient one. 



In contrast to the last, but probably having a primitive 

 common origin with them as forms that all originated in land- 

 locked areas, are the earth and river worms or Oligocha^ta, 

 also the leeches, or Hirudinea. For, if the Polvcha^ta mav 

 well be regarded as an ancient marine group, equally may the 

 two last-named be regarded as terrestrial. 



