100 SoLtas—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 Palemonina usually leave the egg in the zoéa stage, 
but the freshwater in a stage more advanced than the Mysis, as Fritz Miiller shows 
in his description of the development of a Paleemon 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 
larve. 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- 
luses are not known to occur in asingle 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 
Mollusea, the Cephalopods, we find characters which one would expect to render 
* Tn rasping the leaves of freshwater plants the gastropods will necessarily devour hosts of Infusoria. 
Infusoria also help to furnish food for lammellibranchs. 
