Before entering minutely into the description of shells, it will be necessary to distinguish from the true testaceous Mollusca two kinds of animals which have formerly been associated with them. Of these, the first is the class of CRUSTACEA, consisting of crabs, crayfish, &c. These differ from shell-fish, not only in structure and chemical composition, but also in the fact that the animal has jointed limbs, and that the substance of the flesh is inseparable from the hard external covering, which invests each particular joint as with a sheath; whereas the Molluscous animal is but partially attached to its shell, from which it possesses the power of partly withdrawing and returning. The second class is that to which the sea-urchin, or Echinus, belongs, of which there are many genera and species. The testaceous covering of Echini is composed of a number of small pieces, placed edge to edge, forming a more or less globular external covering to the flesh, which is supported in the centre by a number of bones leaning upon each other in a pyramidal form. The _test_ is of a fibrous texture, guarded on the outside with moveable spines, which turn on ball and socket joints. A true shell is composed of one or more calcareous pieces, commonly called valves, each piece formed by a series of layers, applied obliquely upon each other, in such a manner that each new layer begins within, and terminates a little in advance of the one before it. STRUCTURE AND GROWTH. We shall now endeavour to describe the manner in which the growth of each separate valve, or each regularly formed shell, proceeds from the nucleus. Before the young animal has left the egg, if it be an _oviparous_ species, or the body of the parent if _viviparous_, the nucleus of the shell is generally formed, and specimens are sometimes preserved in which the young shell is seen within the egg, as in the cut, fig. 1, 2; or adhering to the inner surface of the full-grown shell by the dried mucus of the animal, as seen in fig. 3. [Illustration: 1. Egg of a Bulinus. 2. The same broken, shewing the young shell. 3. The young of a Paludina, as seen in the aperture of the shell.]