16 EOSTEIFERA. 



habits of the animals : — 1. Platijpoda. The foot 

 depressed, expanded for crawling. 2. Protopoda. 

 Foot roundish, truncate, or clavate when the ani- 

 mals are sedentary, living in shells attached to 

 marine bodies. 3. Leptopoda. Foot compressed for 

 leaping. 4. Heteropoda. The foot compressed, fin- 

 like, with a small sucker for attaching Itself to float- 

 ing bodies. 



The British fluvlatlle and terrestrial species all 

 belong to the first section, and form a division of 

 it, characterised by having the eyes sessile, or nearly 

 so, on the outer side of the bases of the subulate 

 tentacles, and the mantle enclosed in the shell, which 

 have been called Edriophthahna, Cochlea. 



These animals are divided Into groups or families, 

 according to the formation of the respiratory organs 

 or gills, and position of the eyes, thus : — 



1. CyclostomidcB. Eyes lateral, external; gills 



vascular. 



2. LittorinidcE. Eyes lateral, external ; gills laml- 



nal ; operculum spiral. 



3. BltliiniadcE. Eyes lateral, external ; gills lami- 



nar ; operculum annular. 



4. ViviparidcB. Eyes lateral, external ; gills Ji- 



lamentous ; opercvdum annular. 



5. Valvatid<2. Eyes lateral, external ; gills sub- 



pinnate, exposed ; operculum spiral. 



6. Aciculida. Eyes at the internal base of the 



tentacles ; operculum spiral. 



