MODE OF LIFE. 359 



gills, nephridia, &c. According to Lang, during the torsion an increased 

 growth of the upper surface of the visceral mass is necessary in order to 

 avoid rupture, and thus the superficial spiral coiling is produced ; this 

 is reflected in the coiling of the shell. In one series of the Gasteropods 

 the visceral nerve loop, running from the cerebral and pleural to the 

 visceral ganglia, is "caught in the tvi-ist," and twines like a figure 8 

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

 and untwisted (Euthyneural condition). In both groups 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 in- 

 cludes, 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 com- 

 pared 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 {LimncEiis) 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. 



Statistics are neither interesting nor reliable, for there is 

 much difference of opinion as to the 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 Gasteropods are reckoned at about 10,000, 

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

 streptoneural, the relatively small minority are euthyneural 

 opisthobranchs and nudibranchs, with light shells or none. 

 The Heteropods and some opisthobranchs live in the open 

 sea ; the great majority of aquatic Gasteropods frequent the 



