10 MANUAL 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 territory, its appropriate food, and its duties to perform 

 in the economy of nature. Our present purpose, however, is to 

 point out those circumstances and trace the progress of those 

 changes which are not peculiar to individuals or to species, 

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

 class. 



In their infancy the molluscous animals are more alike, both 

 in appearance 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 butterfly. The analogy, however, is reversed 

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

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

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

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

 climate and conditions are 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 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 drifted 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 



