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XXX.— ON THE OEIGIN OF EEESHWATER EAUNAS : A STUDY 

 IN EYOLTJTION. Bt W. J. SOLLAS, M.A:, D.Sc, E.G.S., 



Professor of Geology and Mineralogy in the University of 

 Dublin. (Abstract.) 



[Bead, May 19, 1884.] 



The poverty of freshwater faunas as compared with, marine is 

 commonly attributed to a supposed inadaptability on the part of 

 marine organisms to existence in fresh water. That this explana- 

 tion is altogether inadequate is shown by the existence of fresh- 

 water jelly-fish such as Limnocodium, and still more directly by 

 the experiments of Beudant, who succeeded in accustoming several 

 kinds of marine mollusca to a freshwater habitat. The view of 

 Yon Martens is, that the severity of a freshwater climate is pro- 

 hibitive of the existence of most marine forms in rivers, and espe- 

 cially in the rivers of temperate regions. This cause, though no 

 doubt eflficient, is not altogether sufficient, and a more thorough- 

 going explanation is necessary. This is chiefly to be found in a 

 study of the means by which a wide distribution of marine animals 

 is secured. In the case of stationary forms this is accomplished 

 by means of free-swimming embryos, and as these are transported 

 by currents, they can never pass from the sea into rivers, in which 

 the current is always directed seawards. Nor, probably, could an 

 attached form once introduced into a river permanently establish 

 itself so long as its propagation took place exclusively through 

 free-swimming larvse, for these would gradually be borne out to sea. 

 Hence, freshwater animals should not, as "a rule, pass through a 

 free larval stage of existence, nor as a matter of fact do they. In 

 the Hydra, freshwater Sponges, and^Polyzoa, the young usually 

 emerge from a horny cyst in the complete state. In the TJnio- 

 nidse, the glochidium stage provides for a distribution, in which a 

 journey invariably seawards is not involved. 



Faludina is viviparous, and as in the case of other freshwater 

 molluscs, the young do not enter upon a free existence till they are 

 essentially similar in their mode of ^movement to their parents. 



