NATURAL HISTORY. 
105 
GALLERY.] 
The Table Cases contain the collection of the Shells 
of Molluscous Animals (in progress of arrangement). 
Shells are the hard bodies which are secreted for their 
protection by the surface of certain soft inarticulated 
animals, (called Mollusca;) they are generally large enough 
to cover the whole of the body, but some are so small as 
only to protect the more important organs, as the heart, 
lungs, &c. The shell is formed on the animal before it is 
excluded from the egg, and even before the unhatched 
animal has gained all its organs ; and a few kinds, as 
the Dorides , &c., which are destitute of the shell in their 
adult state, at which period they are covered only with, a 
cartilaginous skin, have a shell to cover their soft and just 
hatched bodies. 
The animals which form these shells constitute a par¬ 
ticular division of the animal kingdom, which, from their 
being soft, fleshy, and destitute of any bony skeleton sup¬ 
porting jointed limbs, or of any hard ringed skin, have been 
called Mollusca. They are covered with a muscular coat, 
called the mantle , endued with a glairy humour, and are 
generally elongate; walking on a single central foot or 
disk, and furnished with one or more pairs of organs on 
the head and sides, to enable them to move from place to 
place; but their most distinctive character is, that their 
nervous system consists of a certain number of medullary 
masses, or ganglions, distributing fibres to different parts 
of the body; one of the masses being placed over the gul¬ 
let, and enveloping it like a collar. 
The shell is formed by the hardening of the animal mat¬ 
ter which is secreted by certain glands on the surface 
of the body, by means of chalky matter, which is also 
secreted by similar glands. The unhatched animal, 
very shortly after it is formed, begins to construct its 
shell; and when the animal is hatched, it deposits on the 
edge of the mouth of the little shell which covered its body 
in the egg, a small quantity of mucous secretion. This dries, 
and is then lined with other mucous matter, intermixed with 
calcareous particles, and when this becomes hardened the ani¬ 
mal again places on its edge another thin layer of the mucous 
secretion, and again lines it as before. The mucous secretion 
first deposited, called Periostraca , forms the outer coat of the 
shell, and is of use in protecting it from injury, while the 
f 3 
