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 " (Paludinida). might be termed freshwater 

 Periwinkles, as the animals of both are very similar. The true 

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



Pig. 1(). 



"A 



The Common British River-Snail (Paludina vivipara). 

 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 Calyptrceides includes the "Slipper-Limpets" [Case 9, 

 (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 " ( Vermetidce) 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 " (Strombicke) are the largest of the Gastropods [Case 9, 

 with a proboscis or non-retractile snout. They do not crawl like E_u -i 

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



