4v 
488 MOLLUSCA. 
the economy of nature. An account of these is their natural history, and this is hkely to prove 
interesting, somewhat in proportion to the peculiarities of the creatures themselves, and the 
novelty of the revelations which their powers, instincts, and habits unfold. 
Throughout all the classes of animals we have described, we have found an internal bony skel- 
eton, forming" the foundation for the whole structure of the body — limbs, flesh, muscles, and 
nerves. In the Molhisca the bodies are soft, and instead of having an internal bony support, 
they are mostly protected by a hard external shell. These soft bodies are enveloped in a 
muscular sldu, which naturalists call the mantle, and it is this which by slow degrees secretes and 
supplies the shell. In some species the shell is of one piece, and is coWq^l univalve ; in others the 
shell is double, the two parts being united by a hinge; this is called bivalve. The snail is a 
univalve, the oyster or clam a bivalve. Other shells, on account of their structure, are called 
multivalve. Many shells, as that of the oyster, are deposited in layers, a fine membrane inter- 
posing between each layer ; they are therefore called membraneous shells. Most membraneous 
shells are lined with a brilliant enameled substance, called nacre; mother of j>earl is the nacre 
of the pearl oyster. That of the fresh-water mussel is a beautiful azure. The other structure of 
shells is called porcellaneous, because they look like porcelain or china. The common cowry is 
a well-knoAvn instance of a porcellaneous shell. Some shells are so transparent as to resemble 
glass, and are therefore called vitreous. 
Starting with these simple definitions, let us take a general survey of the field of inquiry which 
lies before us. We begin with the infancy of these animals, and we may remark that at this 
point in their lives, the various kinds of Mollusca are more alike both in appearance and habits 
than in after-life ; the young fry of the aquatic races are, indeed, almost as different from their 
parents asthe caterpillar from the butterfly. The analogy, however, is reversed in one respect ; for 
whereas tne adult shell-fish are often sedentary, 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 races as far as suita.ble 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 forever increasing over the bed of the deep sea, at depths too 
great for any living thing to inhabit. 
Some of these little creatures shelter themselves beneath the shell of their parent for a time; 
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 appro- 
priate locality. After a few days, or even less, 
of this sportive existence, the sedentary tribes 
settle down in the place they intend to occupy 
during the remainder of their lives. The tuni- 
cary cements itself to rock or sea-weed ; the 
ship-worm adheres to timber, and the pholas 
and lithodomus 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 bracliiopoda are all fixed by similar means, 
and even some of the gasteropods become 
voluntary prisoners, as the hipponyx and ver- 
metus. 
Other tribes retain the power of traveling at will, and shift their quarters periodically, or in 
search of food ; the river-mussel drags itself slowly along by protruding and contracting its flexible 
