Page:Natural History, Mollusca.djvu/89

 “The shells of the newly-hatched animals have been frequently considered as distinct species, and some very thin shells of land Mollusca, such as Vitrinœ, have been taken for the young of other well-known species, as Helix hortensis. These young shells are easily known by their always being of a pale horn colour; the whorls are generally rather irregular, and enlarge very rapidly; and the apex of the whorl, which was first formed, is generally large and blunt compared with the size of the shell. They are always destitute of colour, for the animal does not deposit the colouring matter until after it has been hatched; and it is therefore generally easy to distinguish, in the young shell, (and sometimes also in the adult,) that part of the top of the spire which formed the shell of the animal when in the egg.

“The shell is formed by the hardening of the animal matter, which is secreted by certain glands on the surface of the body, by means of chalky matter, which is also secreted by similar glands. It has been stated that the unhatched animal, very shortly after it is formed, begins to make its shell; and when it is hatched, deposits on the edge of the mouth of the little shell, which covered its body in the egg, a small quantity of the mucous secretion. This dries, and is then lined with some mucous matter, intermixed with calcareous particles; and when this hardens, it again places on its edge another very thin layer of the mucous secretion, and again lines it as before. The mucous secretion first deposited forms the outer coat of the shell, and is of use in protecting it from injury, while the mucous matter mixed with lime, which is placed within it, forms the substance of the shell itself.