HABITS AND ECONOMY OF THE MOLLUSCA. % 
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 HCONOMY OF THE MOLLUSOA. 
Hyery 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 pecular to individuals 
or to species, but haye 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 climato 
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 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 ayoid being drifted 
away. ‘They all have a protecting shell, and even the young 
bivalves haye 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 
~holas and lithodomus to limestone rocks, in which they soon 
excayate 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 brachiopoda are all fixed by similar means, and eyen some 
of the gasteropods become yoluntary prisoners, as the hipponya 
and vermetus. 
