98 Sottas—On the Origin of Freshwater Faunas. — 
flapping of their valves, attach themselves, like external parasites, to the animal 
whose presence has stimulated their efforts. Becoming encysted by an epidermic 
outgrowth of the host, they are carried about till they become metamorphosed into 
young Anodons or Unios; and then, escaping from the cyst, drop to the bottom 
of the stream, and assume a sedentary mode of life. In this way they become 
dispersed through the river they inhabit; and when transported by birds may be 
transferred from one river system to another. 
Of the next family of Lammellibranchiata, the Cyprinacea, only three genera 
are river-dwellers, Cyrena, Cyclas, and Pisidium. In Pisidium the development 
of the embryo proceeds within the gill-plates of the mother; so that in this case 
also the young larve are secured from a seaward journey. The development of 
Cyclas is also intraparental ; and no free larval stage exists. 
With respect to the mode ‘of development of the few forms which, among the 
Veneracea, are found inhabiting rivers, I can obtain no information. 
Passing to the Mytilacea, we find the freshwater Dreissena, common in some 
English rivers and canals, into which it has been imported within recent times. 
Although I can find no details respecting its development,* this need not trouble 
us, as its habit of attaching itself to floating objects will quite readily account 
for its introduction. I can find no reference to the development of the Mulleracea. 
So far as our imperfect examination of the Lammellibranchs permits, we may 
claim this group as affording evidence in support of our hypothesis. 
GASTROPODA. 
STREPTONEURA. | EUTHYNEURA. 
Rhaprdoglossa. | Pulmonata. 
Neritidee, fm. 
Teenioglossa Basommatophora. 
Melaniide, 0s Limneeide, fs 05 
Entoconchide. 0. mM. Auriculide. brackish water. 
Littorinide, fim. 
Palednids fio Stylommatophora. 
ee, . 0. 
Valvatide, UnG: Oncidiada. salt marshes, t. 
Ampullaride. Fao: 
Nerita is a marine genus; but it contains species which dwell in freshwater 
streams. One species, in the Philippines, climbs trees. 
Neritina is a freshwater genus; but JV. fluviatilis, which is found in British 
rivers, also occurs in the brackish waters of the Baltic; while WV. wiridis and WN. 
meleagris are Indian species which live in the sea. 
* Prof. Lankester thinks it probable that the larva of Dreissena is free-swimming, and that both it 
and Cordylophora are lacustrine rather than fluviatile forms. 
