10 MANUAL OF THE MOLLUSCA. 



Chapter III. f 



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 territory, its appropriate food, and its duties to perform 

 in the economy of natm-e. Our present pm-pose, however, is to j 

 point out those cii'cumstances and trace the progress of those ; 

 changes which are not peculiar to individuals or to species, 

 but have a wider application, and fonn the history of a great j 

 class. ' 



In their infancy the molluscous animals are more alike, both ] 

 in appeai'ance and habits, than in after life ; and the fiy of the I 

 acquatic races are almost as different from their parents as the I 

 caterpillar from the butterfly. The analogy, however, is reversed i 

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

 tary, or walk with becoming gravity, the young are all swimmers, i 

 and by means of their fins and the ocean-currents, they travel i 

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

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

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

 their tiny and fragile shells become pai't of a deposit that is for i 

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

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



Some of these little 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, j 

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

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

 appropriate locality. | 



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



