Page:The American Cyclopædia (1879) Volume V.djvu/204

 200 CONCHOLOGY But though the important divisions of the mol- lusca are now based on their internal organi- zation, the study of the shells is nevertheless not to be neglected in the pursuit of the gene- ral science of malacology, or the study of the mollusca ; for the minor divisions into species are still generally controlled by differences in the forms of the shell ; and in the important department of pala3 ontology, it is only the outer coverings of the ancient mollusca, or casts of these in more durable materials, that are pre- sented to us, by which we may determine their character. Shells consist of carbonate of lime secreted by the animal, and intermixed with some animal matter ; they are usually heavier where calcareous formations prevail, the mol- lusca which bear them also being more abun- dant than in granitic and other districts defi- cient in lime. The secretion is first of an al- buminous matter through the pores of the outer mantle of the embryo mollusk ; this is generally succeeded by an admixture of crystalline par- ticles of carbonate of lime ; new layers form beneath these, and the shell gradually gains thickness and extension. In the species in which it is least developed it appears as a hol- low cone or plate, which serves as a protec- tion to the breathing organ and heart. The mantle it covers in other species is in part the same important organ. The protuberances and ridges seen on many univalve and bivalve shells appear in the course of their growth by the margin of the mantle turning out at a con- siderable angle, and thus building up a plate in this position for a certain distance. This growth then ceases, the mantle retracts, or may be regarded as changing itself into the shelly layers, and then it extends in the origi- nal direction, carrying out the shell with it, till it turns again to form a second plate or ridge ; and so the process goes on. The' spines upon shells are produced by the mantle sending out from its margin tentacula, which secrete from their surface the calcareous matter, and thus form slender tubes. These become solid, as the fleshy matter, while it retracts and is absorbed, continues still to deposit the shelly layers. As each successive layer in the growth of shells extends beyond the line of the next older layer, the edges of these must appear upon the outer surface as marked lines of growth. Many mol- lusks possess the power of altering and enlarg- ing their shells to adapt them to their growth, which they appear to do as if by an intelligent will. Thus the murices remove the spines that interfere with the extension of the shell ; and others enlarge the aperture, when it has become too contracted for the body, by wearing away or dissolving its walls. The growing cyprcea removes the inner wall of its shell, and extend- ing its mantle around upon the outside, adds there, in successive layers, more than was taken from within. The shell not only thus changes in size,but the form of the adult becomes alto- gether different from that of the young. The distinguishing marks of shells are the number of parts of which they are composed, and their peculiar forms and prominences. They possess also different textures and various colors, often of great beauty. Some consist of a single piece, as those univalves which are not provided with the horny or calcareous operculum, such as is attached to the posterior part of the foot of other species, and which, when the foot is drawn in, serves to close the aperture as with a tight and perfectly fitting cover. In these the shell is in two pieces; so also is it in most of the bivalves, and the resemblance of the arrangement has led some conchologists to suppose that the operculum may be regarded as the equivalent of the dextral valve of the con- chifera; but the anatomical structure forbids this conclusion. In the terebratula the shell is in three parts, and in some of the pholades and chitons it is in four or more. The peculiar forms of the aperture in some of the univalves are found to bear constant relations to the in- ternal organization of the animal, and so far are therefore to be relied on as indicative of its habits and character. Such are the grooved and notched siphons in some of the spiral uni- valves peculiar to marine mollusca, which breathe by two gills furnished with a fleshy tube for supplying to them the water. The textures of shells are described as porcellanous, pearly or nacreous, fibrous, horny, and some are glassy and translucent. These textures are produced by the different manner in which the particles of mineral and animal matter are arranged, which is best exposed when the shells are partially decomposed by the disap- pearance of the animal substance, a condition often met with in fossil shells. In some of them the complicated arrangement of the three strata of plates, which make up the porcella- nous structure, is exhibited in the broken edges of the cyprseas, cones, &c. The outer and inner layers present the edges of the Slates to the surface, while those of the mid- le one lie in groups at different angles. Each plate is composed of minute prismatic cells, arranged at angles of 45 with the plane of the plate, and these meet the cells of the adjoining plates at right angles. The animal matter of the porcellanous shells is soluble gelatine. The pearly shells are in alternating layers of very thin albuminous membrane and carbonate of lime, which by their minute un- dulations give the pearly lustre. This struc- ture is the least permanent, and in some geo- logical formations the shells that were provided with it have disappeared, leaving only their casts, while those of fibrous texture are pre- served unchanged. This texture is produced by an arrangement of prismatic cells of car- bonate of lime, which extend continuously through the successive layers that make up the shell. It is well exhibited in the heavy fossil inoceramus, which sometimes is found falling into fibrous blocks, the grain transverse to the surface of the shell. The oyster shell, in which the prismatic cells are irregularly ar-