SOLLAS On the Origin of Freshwater Faunas. 101 



them eminently adapted to a freshwater inodo of existence, such as their firm and 

 compact tissues. remarkable activity, and emergence from the egg in a complete 

 state. Vet mi such animal as a freshwater cuttlefish is known. In this case a 

 deficiency of appropriate food is prohalily the efficient harrier, for these molluscs 

 are exceedingly voracious, feeding largely on Crustacea they have been known 

 to overpower the lohster in fair ti^ht and the freshwater Crustacea of oar; strQanw 

 are prohalily not numerous enough to furnish them with a dependable .meat supply.' 

 Like tlie Murexes, they may also be regarded as amongst the dominant- cliisaej' o/; 

 the Mollusca, and, being pre-eminently successful in the struggle for existence, are 

 not forcilily j)iished into rivers for a means of subsistence, so that supposing immi- 

 gration poil>le, no sufficiently strong reason for it exists. The habits of animals 

 must al-o lie taken into account, for these, in many cases, seem to be as character- 

 istic of the species as are structural peculiarities, and a change from 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, t. e. 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 



) the area which they inhabit may be converted into a freshwater basin or 



lake ; (.'{) they may acquire a terrestrial or marsh-loving habitat, and subsequently 



iiange 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 Oligochseta, as Professor Haddon has 

 siiL r Lre>ted to me. 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 Tubularidje among the Hydrozoa pass through a freely- 

 moving actinula stage; yet, as this is transitory, it is scarcely competent to explain 

 the presence of Hvdra in our ponds.* Such locomotive 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 geologic 

 times, have persisted throughout life in the ancestral Tubularidne, 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. 



