GASTROPODA. 



21 



about 100 recent and 200 fossil species are known. One species only 

 ( Turritella communis) is now found living on the British coasts. 



The " River-Snails " (Paludinidce) might be termed freshwater [Case 9, 

 Periwinkles, as the animals of both are very similar. The true A ~ B -J 

 Paludince are viviparous. They are rather sluggish, and found at 



Fio. 10. 



M 



[Case 9, 



B-C.J 



The Common British River-Snail (Paludina wvipard). 



a, head ; b, tentacles ; c, eyes ; d, foot ; e, operculum. 



the bottom of ponds and rivers feeding on decaying animal and 

 vegetable matter. 



The family of Calyptrmdce includes the " Slipper- Limpets " 

 (Crepidula) and the " Cup-and-saucer Limpets" (Crucibulum) . 

 Although furnished with a foot, they rarely crawl about, but 

 remain attached to rocks, stones, or other shells, sometimes 

 forming a shelly plate under the foot by which they become fixed 

 to the spot where they have taken up their abode. 



The " Worm-shells " ( Vermetida) are a very peculiar family. [Case 9, 

 Their shells can scarcely be distinguished from the shelly tubes 

 which are formed by certain species of marine worms, Serpula, &c. 

 They are free and spiral in early life, but afterwards become 

 distorted and generally attached to rocks, stones, &c. A foot for 

 walking purposes therefore would be of no use ; consequently it 

 is more or less obsolete, serving only as a support to the 

 operculum. 



The " Wing-shells " (Strombidce) are the largest of the Gastropods [Case 9, 

 with a proboscis or non-retractile snout. They do not crawl like E_H- J 

 most other Gastropods, but progress by a sort of hopping move- 



