FRESHWATER JELLY-FISH 211 



coast of Trinidad, how produced he does not say, which 

 communicate at times with the sea, and then receive a 

 number of marine inhabitants. When subsequently the 

 water of these lagoons becomes fresh, several of these 

 animals, adapting themselves to the changed conditions, 

 continue to live on, and amongst these is a jelly-fish, of 

 which Dr. von Kennel tells us nothing further, except 

 that it is not a Limnocodium. The transformation of a 

 marine jelly-fish into a freshwater one has thus been 

 brought about in a very simple manner, without invoking 

 the tremendous machinery of earth movements. This, 

 however, is only the first step to account for the presence 

 of a jelly-fish in the interior of a continent ; it must next 

 be shown that the creature is capable of making its way 

 up the course of a river, and on this point we have little 

 evidence. A great jelly-fish, Crambessa Tagi, is described 

 by Haeckel as ascending the Tagus till it reaches quite 

 fresh water, and other instances of a similar kind are on 

 record. In considering the possibilities of the case we 

 should neglect none, however unlikely it may appear at 

 first sight, and there is one which we must not overlook. 

 Although Limnocnida is not known to pass through the 

 hydroid stage now, yet it may once have done so ; 

 Limnocodium still does, as we know from the descriptions 

 of Dr. A. G. Bourne.* If, while in the hydroid stage, it 

 grew attached to the outer skeleton of some actively 

 locomotive animal, such, for instance, as one of the 

 reptiles which abounded in Mesozoic times, and even at 

 a later date, then, on the further supposition that its 

 host sometimes made excursions from the sea into fresh 

 water, we should have a means by which the hydroid 

 might be introduced. The natural objection to this 

 hypothesis will be found in the danger attending a rapid 

 transference from a salt to a freshwater medium, and 

 * Proc. Hoy. Soc.. vol. xxxviii., 1884, p. 9. 



