9. VERMES, _Worms_. With lengthened body without divisions;
for instance, worms found in the intestines.
10. RADIARIÆ, (_radiated animals_,) with the different parts
of which they are formed arranged like rays round one
common centre; as, for instance, the Star-fish.
11. POLYPI, (_many feet_.) The animal that forms the Coral
is a Polypus.
12. INFUSORIA, (_Infusory animals_.) These are found in
infusions of vegetable substances in water, and are,
in general, too small to be visible to the naked eye.
In the present little book we shall treat of the first five of these
Classes.
The Molluscous animals are, on account of their organization, placed
first among the invertebral animals, a few of the species resembling,
in some respects, the more simply-formed fishes.
The systematic arrangement of the Molluscous animals, considered
not only as regards their shells, but having reference also to the
anatomical distinctions of the creatures themselves, is a modern study.
In ancient authors we merely find a few scattered facts, the beauty of
the shells attracting their notice more than the value or nature of the
animals.
Although, at the first glance, the inhabitants of shells appear to
be beings of a very uninteresting nature, a due consideration of the
valuable properties of many, and the usefulness of all, will enable
us to perceive, that, regarding them merely in an interested point of
view, they are worthy of the strictest attention of the naturalist.
In the first place, the whole of them afford food for the different
species of fish, and other inhabitants of the deep. The Tyrian dye, the
royal purple of the ancients, was produced by the inhabitant of a small
univalve shell, of the genus Purpura. That beautiful ornament in dress,
the oriental pearl, is the consequence of disease in a species of
mussel, and the inner portion of the shell of the same animal, is the
well-known substance, mother-of-pearl. A kind of silk is obtained from
the beard of the pinna, which, in some places, is made into gloves. As
an article of food we may mention the well-known oyster, the mussel,
scallop, &c., and some of the larger kinds form no small portion of
the subsistence of the natives of the South-Sea Islands, and the Negro
population in the West Indies.
The _Teredo navalis_, or ship-worm, has, by its destructive powers,
ruined the noblest vessels, and rendered useless the timbers, on which
many of the constructions in harbours mainly depend for security; on
this account great attention has been bestowed on its natural history
and habits. The barnacle, which attaches itself to the bottoms of
ships, renders the planks so foul, as to interfere materially with
the rate of sailing of the vessel itself. These are only a few of the
useful and noxious qualities of these inhabitants of the deep.
The shell with which a Molluscous animal is covered, is absolutely
necessary to protect its delicate body from injury; this shell is, in
general, composed of much the same substances as bone; but the bone of
a bird, or quadruped, is formed by the agency of the blood, and the
particles of which it is composed are deposited by that fluid, and
again taken up and restored to the circulation, a circumstance which
does not take place in the substance of a shell. The shell is formed
by the deposition of layer upon layer, in the course of the growth of
the animal, and the ridges we perceive on many shells, point out their
periodical increase.
[Illustration: _Fig 1. Fig 2._]
It will be necessary, when describing the distinctions between shells
of different genera, to use several terms, which will, unless properly
defined, be, perhaps, unintelligible to young people. The annexed
diagrams will explain the meaning of those of most frequent occurrence
among the Mollusca. Fig. 1, represents a univalve shell; fig. 2,
another shell, of the same division, cut through the middle, for the
purpose of showing the columella, or pillar. Many shells, as, for
instance, the periwinkle, (_Turbo vulgaris_,) have what is called
an operculum, (_a lid_,) which closes the opening, and protects the
inhabitant from injury. In the case of the periwinkle, this lid is of
horny nature, but, in many species it is hard and solid, like the shell
itself.