. Animal life as affected by the natural conditions of existence. Animal ecology. STRUCTUKE OF SHELLS. 213 their growth; and it is plain that if freely creeping or swimming creatures are exposed to it they will probably in most cases be destroyed. Sometimes, however, these are capable of escaping its effects by their own strength. This occurs, for instance, in the case of many MoUusca, of which the shells are often much injured by erosion. All conchologists are well aware that the shells of fresh-water MoUusca in particular are generally more or less eroded; in. many places it is very difficul
. Animal life as affected by the natural conditions of existence. Animal ecology. STRUCTUKE OF SHELLS. 213 their growth; and it is plain that if freely creeping or swimming creatures are exposed to it they will probably in most cases be destroyed. Sometimes, however, these are capable of escaping its effects by their own strength. This occurs, for instance, in the case of many MoUusca, of which the shells are often much injured by erosion. All conchologists are well aware that the shells of fresh-water MoUusca in particular are generally more or less eroded; in. many places it is very difficult to find any number of certain species of shells that are not thus worn away, and the process commonly begins at the apex of the shell. It can hardly be doubted that there are very various causes for this ; in many cases, which I myself have observed and studied minutely,. Fig. 62.—Oblique section through the shell of a fresh-water mussel, Unio. a, Cuticle, ex- terual and ort^anlc; b, the FrlsniEtic Layer next to it; c, Hacreous Layer, slightly magDified. two causes have combined—namely, the boring powers of the filaments of certain Fungi and the constant wear of fresh- water currents, by which, in the Philippines, the shells of various species of Melania, NaviceUa, and Neritina are attacked. How this occurs can only be understood by briefly studying the structure of such shells. In the shells of Bivalves as well as in those of Univalves three typical layers of structure may be distinguished : the outer one consists invariably of purely organic matter, known to conchologists as the epidermis; the two inner layers consist of calcic carbonate, combined with a very small quantity of feebly developed organic matter. The outer layer of these two is commonly designated as the prismatic layer; the inner one as the mother-of-pearl or nacreous ;" In most of the MoUusca living in fresh water the external organic layer. Please note that these images are extracted from
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