SOLLAS On the Origin of Freshwater f'uiinat. '.'.> 



Littorina is a marine genus; but it contains two sub-genera, 

 (S. K. Kuropc, and Central Africa, L. lan(/<int/ikit)iLud Limnotrochus (L. taii</<tn>jika\ 

 which inhabit fresh water. 



Compared with the rich variety of forms which they present in other parts of 

 the world, tlio freshwater Casteropoda are but poorly represented in our British 

 streams. Tin- chief genera are Limnea, Ancylus, Physa, Planorbis, and Valvata, 

 bi'l'Mi^ini: to the eiithyneurous Gasteropoda, and Paludina and Bithynia, belonging 

 to the Strcptniieura. 



Although not quite germane to our subject, we may stop to point out tin 

 remarkable thinne>s of the shell in these molluscs, and its frequent correlation with 

 a thick epidermis (periostr.icum). The thinness of shell is suggestive at first sight 

 either of a deficiency of carbonate of lime in fresh water, or of greater difficulty in 

 extricating it from u solution in which sodium chloride is scarcely present. 

 Analvses, however. >how that fresh water is usually by no means deficient as com- 

 pared with sea water in calcium carbonate; and the fact that the Unionidae frequently 

 quite massive shells indicates that the absence of sodium chloride has no 

 appreciable effect. The erosion of the umbones of Unio shells and earlier-formed 

 whorls in Paludinre seems to suggest an excess of free carbonic acid, and perhaps 

 other acids which might tend to hinder the secretion of solid shell: and possibly 

 the thick epidermis not only represents layers which in a marine shell would be 

 calcified, but also functionally serves to protect the already formed shell from solu- 

 tion.* On the whole it seems most probable that the thin shell has arisen by 

 natural selection, and is correlated with the lower specific gravity of fresh water as 

 compared to salt. This diminished density would render it needful for the free 

 nioveiiii-ii's of the animal that it should be disburdened of all unnecessary weight. 

 On the other hand, the thicker shells of marine mollusca, so often found broken, are 

 probably correlated with the occurrence of powerful shifting currents and storm 

 waves in the medium which they inhabit. f 



Painir on to our main inquiry, we find that the euthyneurous Gasteropoda 

 usually deposit their eggs in jelly, which is attached to some foreign object, and 

 the young emerge in a fully formed state. In the streptoneurous division we have 

 Bithvnia and Paludina: the former attaches its eggs, but the latter and this is a 

 significant fact is viviparous, the young molluscs leaving the mantle chamber of 

 the parent as young adults. 



The Crustacea furnish us with abundance of freshwater forms, Copepods, 

 Branchiopods, and Ostrapods, Isopods and Amphipods; and Pahemonintr, and 



aeina- among the Decapods. But the first three Croups contain none but minute 

 forms, which are capable of distribution by birds and probably winds. They 



Semper has already suggested that the epidermis serves this purpose. 



t Some interesting remarks bearing on this subject will b>> found in a Paper by Mr. A. R. Hunt, in 

 thr Proceedings of th,- Hoyal Socitty, London, 1882, p. 8 (reprint). 



