XXxii . INTRODUCTION. [CH. 



of the marine Mollusca are parasitic, or live upon the 

 secretions of other animals. Among the bivalves Mon- 

 tacuta substriata, and among the univalves Stylifer Tur- 

 toni, are instances of these parasitic habits. 



Size. This appears to depend on the supply of food ; 

 and it is probable that, owing to the vast shoals of mol- 

 lusk-eating fish which abound in the northern seas, the 

 shell-fish there are thinned to such an extent that the 

 fortunate survivors have a proportionally larger share of 

 food than those which inhabit southern seas, where both 

 species and individuals are more numerous. Northern 

 Mollusca are generally larger than those of the same 

 species from the South*. The same law is observable 

 with regard to cultivated fruits, thinning being resorted 

 to for the purpose of reducing the number and in- 

 creasing the size of those which are allowed to remain. 



Habitat. There is probably not a square foot of land, 

 either in a cultivated or uncultivated state, or co- 

 vered with fresh, brackish, or salt water, that is not 

 inhabited by Mollusca of various kinds. Trees, herbage, 

 and sea- weeds are the chosen stations of many sorts, 

 for the sake of the shelter or food which they afford ; 

 and even our cellars and kitchens are not free from 

 them. Some live only in the ocean, and never approach 

 land unless when driven on shore by the winds and 

 waves. These are called " pelagic." Among them are 

 the lanthina, or " blue-snail," and a few small Ptero- 

 pods, which are occasionally found in our seas. The 

 former is provided with a cellular apparatus, by which 



* See also Draparnaud (Tabl. Moll. p. 35) as to the comparative size 

 of land shells inhabiting the North and South of France. " Le climat 

 influe beaucoup sur la grandeur des individus. Chez les Mollusques, 

 comme chez les Mousses et un grand nombre d'autres plantes, la meme 

 espece acquiert un volume d'autant moindre, qu'elle habite dans un 

 pays plus chaud." 



