98 



SOLLAS 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 larvae 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. 

 Rhipidoglossa. 



Neritidae, / m. 

 Tcenioglossa. 



Melaniidas, / o. 



Entoconchidae. o. m. 



Littorinidae, /. m. 



Paludinidse, /. o. 



Valvatidae, /. o. 



Ampullaridas. f. o. 



EUTHYNEURA. 



Pulmonata. 



Basommatophora. 



Limnaeidae, 

 Auriculidse. 



Stylommatophora. 

 Oncidiada. 



f.o. 



brackish water. 



salt marshes, t. 



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 N. fluviatilis, which is found in British 

 rivers, also occurs in the brackish waters of the Baltic ; while N. viridis and N. 

 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. 



