Sottas—On the Origin of Freshwater Faunas. 101 
them eminently adapted to a freshwater mode of existence, such as their firm and 
compact tissues, remarkable activity, and emergence from the egg in a complete 
state. Yet no such animal as a freshwater cuttlefish is known. In this case a 
deficiency of appropriate food is probably the efficient barrier, for these molluscs 
are exceedingly voracious, feeding largely on Crustacea—they have been known 
to overpower the lobster in fair fight—and the freshwater Crustacea of our streams 
are probably not numerous enough to furnish them with a dependable meat supply. 
Like the Murexes, they may also be regarded as amongst the dominant classes of 
the Mollusca, and, being pre-eminently successful in the struggle for existence, are 
not forcibly pushed into rivers for a means of subsistence, so that supposing immi- 
gration possible, no sufficiently strong reason for it exists. The habits of animals 
must also be taken into account, for these, in many cases, seem to be as character- 
istic of the species as are structural peculiarities, and a changefrom a marine to 
a freshwater life would therefore only take place under the action of some unusu- 
ally powerful impelling cause. 
Some other impediments to this transformation will be attended to in discussing 
the next part of our inquiry, 7. ¢. as to the mode in which freshwater forms have 
originated, and the circumstances under which some of their chief peculiarities 
have been produced. 
There are at least three conceivable ways by which freshwater animals may be 
derived from marine. The latter may—(1) directly migrate into rivers from the 
sea ; (2) the area which they inhabit may be converted into a freshwater basin or 
lake ; (3) they may acquire a terrestrial or marsh-loving habitat, and subsequently 
exchange this for a fluviatile or lacustrine one. Some of our freshwater Gastropods, 
viz: the freshwater Pulmonata, have most probably acquired their present habitat in 
this circuitous fashion, as also the freshwater Oligocheta, as Professor Haddon has 
suggested tome. We shall now confine ourselves to a discussion of the first two 
alternatives. 
With regard to direct emigration from the sea, we may safely dismiss it as an 
explanation in the case of fixed forms which are not parasitic nor attached to 
locomotive animals. Such forms as Sponges, Polyzoa, and Hydra, are not likely to 
have travelled direct from sea to rivers. Only one genus of locomotive Polyzoon 
is known; and though the Tubularide among the Hydrozoa pass through a freely- 
moving actinula stage; yet, as this is transitory, it is scarcely competent to explain 
the presence of Hydra in our ponds.* Suchlocomotive Mollusca and Crustacea as fulfil 
the three conditions already laid down might, on the other hand, be fairly expected 
to furnish us with instances of direct colonization. Some such possibly exist, but 
* On the other hand, the actinula stage—or one not far removed from it—may, in very early geologie 
times, have persisted throughout life in the ancestral Tubularide, and thus have rendered direct immi- 
gration possible. Certain features in the development of Hydra have been regarded, however, as 
indicating that it is a degraded form of Tubularian, and not a persistent early type. 
