100 SOLLAS On the Origin of Freshwater Faunas. 



pass through a nauplius stage ; but Nauplii are strong swimmers, and quite 

 capable of maintaining their position amidst the slowly-moving water in which 

 they teem. 



The Amphipods carry their ova about with them, tucked under the abdomen; 

 and the segments and limbs are all formed before hatching. 



The marine forms of the Palsemonina usually leave the egg in the zoea stage, 

 but the freshwater in a stage more advanced than the Mysis, as Fritz Miiller shows 

 in his description of the development of a Palaemon living in brooks in Blumenau. 



Astacus carries its eggs about, attached to the swimmerets of its abdomen, like 

 the lobster. The young, however, unlike those of the lobster, are provided with 

 sharply -hooked claws, by which they can maintain a hold of the parent after they 

 have entered upon a free existence. Whether this contrivance is to prevent their 

 being swept away by the river, or to afford them, when necessary, maternal pro- 

 tection, like that of a hen for her chickens, seems doubtful. 



We have now passed in review many of the chief peculiarities in the mode of 

 development of the members of our freshwater fauna, and find, with a few excep- 

 tional cases susceptible of ready explanation, everywhere consistent evidence in 

 favour of our original proposition, that the invertebrate animals inhabiting fresh 

 water might be expected not to propagate exclusively by means of free-swimming 

 larvae. The passage through a free larval stage in the course of development may 

 be regarded as a real explanation of the exclusion of such marine forms as undergo 

 it from a freshwater habitat. 



Three causes are therefore admitted as leading to this exclusion : they are 

 (1) the difference in chemical composition of the medium ; (2) the severe cha- 

 racter of the freshwater climate ; (3) the necessity for the suppression of a free 

 larval existence. The number of animals which can satisfy all these three condi- 

 tions might be expected to be few ; and probably there are other quite as important 

 deterrent causes to be revealed. The absence of suitable food in freshwater 

 streams has been suggested by Semper as one of these, and does probably lead to 

 the exclusion of many marine forms. Thus none of the Gastropods of our streams 

 are carnivorous, in the sense of preying upon other animals of more than microsco- 

 pic size,* and yet many carnivorous Gastropods are actively locomotive, abound 

 on our coasts between tide marks, endure a rigorous climate, and so dispose their 

 eggs as to preserve them, during development, from destruction or transporta- 

 tion by currents. Purpura is a case in point, and Nassa another, yet these mol- 

 luscs are not known to occur in a single freshwater stream. The Murexes, some of 

 which are viviparous, and have no severe climate to contend against in the locali- 

 ties where they occur, are likewise exclusively marine. In another order of 

 Mollusca, the Cephalopods, we find characters which one would expect to render 



* In rasping the leaves of freshwater plants the gastropoda will necessarily devour hosts of Infusoria. 

 Infusoria also help to furnish food for lammellibranchs. 



