422 
PHYLUM MOLLUSCA. 
Of this myriad, about gooo are streptoneural, the relatively 
small minority are euthyneural Opisthobranchs and Nudi- 
branchs, with light shells or none. 
The Heteropods and 
some Opisthobranchs live in the open sea; the great 
majority of aquatic Gasteropods frequent the shore and 
the sea bottom at relatively slight depths; the deep-sea 
forms are comparatively few. 
Gasteropods rarely feed at such a low level as bivalves do 
Fic. 230, —Stages in mol- 
luscan development. 
A., Blastula of limpet (after 
Patten). £, Gastrula_ of 
Paludina vivipara (after 
Tonniges); v., beginning of 
velum; a@vc., archenteron ; 
m., mesoderm cells. C, 
later stage of the same; w, 
velum; 7#., mouth inva- 
gination; a7c., _archen- 
teron; a., anus; f, begin- 
ning of foot; sh.g., shell 
gland. 
—indeed, some of them are fond of 
eating bivalves. Most Prosobranchs 
(streptoneural), with a respiratory 
siphon and a shell notch in which 
this lies, are carnivorous, e.g. the 
buckies (Buccinum) and “dog-whelks” 
(Purpura); on the other hand, those 
without this siphon, and with an 
unnotched shell mouth, feed on 
plants, ¢g. the seaweed-eating peri- 
winkles (Litforina). Most land 
snails and slugs are vegetarian. 
Many Gasteropods, both marine and 
terrestrial, are voracious and indis- 
criminate in their meals; others are 
as markedly specialists or epicures. 
Some marine forms partial to Echino- 
derms have a salivary secretion of 
dilute sulphuric acid, which changes 
the carbonate of lime in the starfish 
into the more brittle and readily 
pulverised sulphate. About ten 
genera are parasitic on or in Echino- 
derms, e.g. Stylifer, Turtonia, Thyca, 
and the extremely degenerate Ex/o- 
concha, within the Holothurian Syz- 
afta. Some species of Lu/ima also 
live a semi-parasitic life on certain 
Echinoderms. 
Life history.—The eggs of Gasteropods are usually small, 
without much yolk, but surrounded by a jelly, the surface 
of which often hardens. 
is an egg-shell of lime. 
In the snail and some others there 
