10 ma:nual of the mollusca. 



Chapter III. 



HABITS AND ECONOMY OF THE MOLLUSCA. 



Every living creature has a history of its own ; each has charac- 

 teristics by which it may be known from its relatives ; each has 

 its own territoiy, its appropriate food, and its duties to perform 

 in the economy of natm-e. Our present purpose, however, is to 

 point out those circumstances and trace the progress of those 

 changes which ai'e not peculiar to individuals or to species, 

 but have a wider application, and form the histoiy of a great 

 class. 



In their infancy the molluscous animals are more alike, both 

 in appeai-ance and habits, than in after life ; and the fry of the 

 acquatic races are almost as different from their parents as the 

 caterpillar from the buttei-fly. The analogy, however, is reversed 

 in one respect ; for whereas the adult shell-fish are often seden- 

 tary, or walk with becoming gi'avity, the young are all swimmers, 

 and by means of their fins and the ocean-cm-rents, they travel 

 to long distances, and thus diffuse their race as far as a suitable 

 climate and conditions ai'c found. Myriads of these little 

 voyagers drift from the shores into the open sea and there perish ; 

 their tiny and fragile shells become part of a deposit that is for 

 ever increasing over the bed of the deep sea, — at depths too 

 great for any living thing to inhabit. (Forbes.) 



Some of these httle creatures shelter themselves beneath the 

 shell of their parent for a time, and many can spin silken threads 

 with which they moor themselves, and avoid being diifted away. 

 They all have a protecting shell, and even the young bivalves 

 have eyes at this period of their lives, to aid them in choosing an 

 appropriate locality. 



After a few days, or even less, of this sportive existence, the 



