MODE OF LIFE. 375 



(Streptoneural condition) ; in the others, the same visceral loop is short 

 and untwisted (Euthyneural condition). In both gioups we find forms 

 with coiled shells, but among the Euthyneura there is a tendency to 

 lose the shell, the visceral hump becoming at the same time incon- 

 spicuous, while a superficial appearance of symmetry is produced. The 

 deep-seated torsion of the organs is, however, still retained. 



It is not very uncommon to find, either as a constant occurrence or as 

 an occasional variation, spirally coiled shells with a reversed or left- 

 handed spiral. In some of these cases the superficial coiling of the 

 visceral hump, as well as the deep-seated torsion, is also left-handed ; 

 but in others we find that the internal structure retains the normal 

 arrangement. 



Mode of life. — From the number of diverse types which 

 the class includes, it is evident that few general statements 

 can be made about the life of Gasteropods. We are safe in 

 saying, however, that though the majority are 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 (Limnmus) foot 

 upwards across the surface of the pool, the explorations of 

 the periwinkles 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. 



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 Gasteropods 

 are reckoned at about 10,000, most of which are marine. 

 Of this myriad, about 9000 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 

 — 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 (Buccinuni) and " dog-whelks " (Purpura) ; on the 

 other hand, those without this siphon, and with an un- 



