| growing. The duration of life in most species of the Mollusca is very prolonged, 
| fishes by the cephalopods. Many of the Mollusca afford a nutritious and palatable 
The propagation of the Mollusca is effected by means of eggs, a marvellous 
number of which are generated by some of the tribe; the oyster, for instance, will 
produce upwards of a million young in the course of a twelvemonth. In some 
cases the young emerge from the egg in a perfectly formed state, in others they 
have to pass through a larva stage. As regards development of size, the snails soon 
attain their maximum, while the cephalopods and mussels never seem to leave off 
it having been ascertained that mussels, which had been marked and kept in some 
particular waters, have attained an age of from 70 to 80 years. 
One of the most prominent peculiarities observable in many of the Mollusca 
is the faculty they possess of being able to construct a shell to live in. As 
mentioned above, the matter required for this purpose is secreted by the mantle, 
its chief ingredient being lime. The shell of the various descriptions of snails 
however, which is always more or less spiral in form, is built up on a core 
consisting of organic matter, around which the particles of lime are disposed, often 
exhibiting a beautifully variegated coloring. Both the shells of the true mussels 
consist of two layers, the outer one of which consists of calcareous matter, while the 
inner one, which lies next the body, exhibits a coating of mother of pearl. The 
two shells are connected by an elastic ligament, and can be opened or closed by 
the animal at pleasure by means of a pair of muscles. 
The body of those Mollusks, which are not furnished with any shell, is covered 
with a firm skin, more or less granulated and slimy. 
Water is an indispensable necessary of life with all the Mollusca, even those 
which live on dry land requiring a large amount of moisture for their existence, a 
prolonged drought being fatal to them. Rivers and inland waters are tenanted by 
a great variety of the snail and mussel tribe, the greatest abundance being however 
found in the Ocean, which contains Mollusks in its deepest waters, and there these 
creatures pass their lives, almost the sole occupation of which consists in feeding, 
seldom stirring, and then only slowly, from one spot. 
With the exception of some of the more highly organised cephalopods and 
some of the natatory snails, which propel themselves with a jerking swimming motion 
by alternately contracting and extending their bodies, the locomotion of the Mol- 
lusca consists of a more or less slow creeping. The organ with which this is 
performed consists of the so-called foot; in the case of the snails, an oblong disc, 
which has the appearance of a dilatation of the abdomen. In the case of the 
bivalve mussels the foot assumes a cuneated shape, being in many cases bent round, 
and this is protruded from between the opened shells and enables the animal to 
propel itself slowly along the bottom. 
Upwards of 20,000 varieties of Mollusca are known as being at present in existence, 
though without doubt the more profound depths of the ocean contain a number of 
other forms, at present unknown to us. The number of varieties in the antediluvian 
world was very much larger, a vast number of fossils of these creatures being met 
with in every geological formation. Of cephalopods, for instance, no less than 
1800 fossil varieties have been found, while the whole number known to be in 
existence in the present age only amounts to 218. 
The usefulness of the Mollusca is not to be undervalued. It largely over- 
balances the damage and destruction wrought by some genera, which is indeed 
limited to the eating away of plants and fruit by the snails and the devouring of 
article of food, others yield pearl and mother of pearl, while others again are 
valuable as constituting the food of other useful animals. — 
The Mollusca are divided into six main groups: I. Cephalopods, II. Snails, 
Ill. Mussels, IV. Brachiopods, V. Ascidians, VJ. Bryozoa. These however, especially 
the snails and mussels, are again divided into several sub-families. The Ascidians 
and Bryozoa, which are mostly found attached as polypi to some fixed spot, have 
only recently been included among the Mollusca by modern Naturalists, owing to 
their affinity in development and internal construction. 
