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.

CLASS MOLLUSCA.

In noticing the animal of a univalve shell, the part which more readily attracts the attention is the mantle, which covers the head of the creature, something like a hood; it varies much in form and size in different genera. The eyes, which in the sepia are amazingly large and brilliant, are very minute in most of the other tribes, although they are frequently visible, and would appear, from their formation, to be of little use as organs of sight; indeed, it is supposed, that in the snail they are devoted to the sense of smelling.

The organs of motion in the Mollusca, according to their different form and position, give names to most of the orders; these consist of muscular expansions of the body, by means of which the animal swims or drags itself along the ground. The gills, or breathing apparatus, are situated internally, and communicate with the air or water, by means of a small canal opening outwardly. The mouth is usually concealed from view when the creature is at rest; in some, this organ is furnished with a hard substance, which supplies the place of teeth, while, in others, it is in the form of a projecting tube. The greatest portion of these creatures are produced in the water, the tribes that inhabit that element, exceeding by far those that are to be found upon the land.

The Molluscous animals have been separated into the five following Orders:—