HABITS AlfD ECONOMY OP THE MOLLUSCA. 7 



in the understanding of the subject, -whereas a "dead and 

 arbitrary arrangement " is a perpetual bar to advancement, 

 •' containing in itself no principle of progression." [Coleridge.) 



Habits and Economy or the Molltjsca. 



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

 characteristics 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 

 aquatic 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 ambulatory, the young are all swimmers ; so that 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 ope a sea and there perish ; their 

 tiny and fragile shells become part of a deposit constantly 

 accumulating, even in the deepest parts of the sea. 



Some of these little creatures shelter themselves beneath the 

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

 threads with which to 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 

 sedentary tribes settle in the place they intend to occupy during 

 the remainder of their lives. The tunicary cements itself to 

 rock or sea-weed ; the ship-worm adheres to timber, and the 

 pholas and Uthodomus to limestone rocks, in which they soon 

 excavate a chamber which renders their first means of anchorage 

 unnecessary. The mya and razor-fish burrow in sand or mud ; 

 the mussel and pinna spin a byssus ; the oyster and spondylus 

 attach themselves by spines or leafy expansions of their shell ; 

 the hrachiopoda are all fixed by similar means, and even some 

 of the gasteropods become voluntary prisoners, as the hipponyx 

 and vermetus. 



