Evolution of Animals 385 



The large division of the Coelenterata or Enterocoela pre- 

 sents a most suggestive study. Unquestionably arising during 

 mid- or late-archsean times, and early branching out into a 

 multiplicity of lime-crusted as well probably as soft-bodied 

 species, it is at the present day represented by several thousand 

 species, that are littoral, marine, or pelagic, with the exception 

 of six genera that are fresh-water. These, however, deserve 

 special notice, for they belong to the most primitive living 

 types, and have a geographical distribution that greatly recalls 

 that of the Porifera. Thus the genus Hydra includes 3, pos- 

 sibly 4-5, species that are world-wide, for equally in Europe, 

 North America, tropical South America, Africa, Australia, 

 and New Zealand it has been met with. Microhydray even 

 more simple than the last, is found in eastern North America, 

 while Polypodkan is found in the Volga River. No marine 

 forms approach these in simplicity of structure. Protohydra 

 and Haleremita, though marine, seem closely related, and 

 may well be migrants seaward. 



But the rather more specialized freshwater genus Cordylophora 

 is a type that perfectly unites the Hydroida division or 

 Eleutheroblastese with the littoral or Gymnoblastese. 



The two unique fresh-water medusoid genera Limnocodium 

 and Limnocnida are found, the one probably in the Amazon 

 waters, amid the Victoria lily, the other certainly in Lake 

 Tanganyika and Victoria Nyanza, also in the river Niger of 

 central Africa. The theoiy which explained the presence of 

 the latter as an imported marine type — when Tanganyika 

 was a supposed arm of a Jurassic sea — has been rejected by 

 Boidenger. The production of a medusoid stage by Micro- 

 hydra is equally mteresting. On the other hand the occurrence 

 ^t times of sea anemones "in brackish or almost fresh water 

 of river estuaries" is fair proof of partial migration from a 

 marine to a fresh- water life, since the entire group of the Antho- 

 zoa is littoral or marine at the present day. 



So unless it can be shoA\ai that one or more genera, as simple 

 as Microhydra and Hydra, also about as abundant, exist in 

 the sea, it seems as logical a position to assert that the group 



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