3i6 MOLLUSCS. 



sluggish when compared with Crustaceans, they are active 

 when compared with Lamellibranchs. 



The locomotion effected by the contractions of the 

 muscular foot is usually a leisurely creeping, but there are 

 many gradations between the activity of Heteropods in the 

 open sea, the gliding of fresh-water snails (LimncBus) foot 

 upwards across the surface of the pool, the explorations of 

 buckles (Buccinum) on the sand of the shore, and the 

 extreme passivity of limpets {Patella) which move only for 

 short distances at a time from their resting-places on the 

 rocks. 



Statistics are not very interesting nor reliable, for there is 

 much difference of opinion as to limits of species and 

 varieties, but we may notice that the number of terrestrial 

 snails and slugs, breathing the air directly by means of a 

 pulmonary-chamber, is estimated at over 6000 living species, 

 while the aquatic Gasteropoda are reckoned at about 10,000, 

 most of which are marine. Of this myriad, about 9000 are 

 streptoneural (we may recall the vast array of shells in the 

 museum cases), the relatively small minority are euthy- 

 neural opisthobranchs and nudibranchs, with light shells or 

 none. The Heteropods and some opisthobranchs live in 

 the open-sea; the great majority of aquatic Gasteropoda 

 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, 

 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, witness the 

 buckles {Buccinum) and " dog-whelks " {Purpura), whose 

 voracity you can watch in the shore-pools ; on the other 

 hand, those without this siphon, and with an unnotched 

 shell-mouth, feed on plants, witness the seaweed-eating 

 periwinkles {Littorina). This correlation is not rigorously 

 true, but it suggests reflection as to the plausibly two-sided 

 connection between very vigorous habits and carnivorous 

 diet, more sluggish habits and vegetarian diet, and finally 

 between passivity and a diet of organic ddbris and animal- 

 cules. The vegetarian habits of most land snails and slugs 

 are known to all. Many Gasteropods, both marine and 

 terrestrial, are very voracious and indiscriminate in their 



