MOLLUS'CA, one of the great animal sub-kingdoms, including so wide a range of dis tinct forms that it is difficult to frame a definition that shall be applicable to all of them_ The lowest forms, termed polyzoa (q.v.) or bryozoa, present so strong a resemblance zoophytes, that until recently they were ass•ciated with the latter; whilst, on the other hand, in some of the most highly organized of this sub-kingdom, the cephalopoda, there is a decided approximation towards the vertebrated series, as is shown by the presence, of a rudimentary cartilaginous skeleton, and by a peculiarity in the development of the embryo. The bilateral symmetry of external form which is almost universal in articu lated and vertebrated animals, is here seldom met with; and taking them as a whole,. the mollusca are' characterized by the absence rather than by the presence of any definite form. The bodies of these animals are always of a soft consistence—a property to which they owe their name, which was devised for them by Olivier, before whose time were included in the Vermes of Linnaeus's arrangement. The thell, when it exists, is not to be regarded as an exoskeleton giving attachment to muscles, and the form of the animal, but merely as an appendage designed for the protection of the body from which it derives its shape; indeed, it is only where the body is uncovered a shell, or where the locomotive oro„ns can be projected beyond it, that any active move-. ments can be effected. The whole fabric is inclosed in a thick, soft, flexible-skin, called. the include, and it is on the surface of this envelope that the shell is formed by the devel opment and subsequent calcification of epithelial cells. In many of the mollusca the shell is composed of a single piece, which is usually a spiral tube, closed at one end, and gradually increasing in size towards the open extremity, from which the animal is able to protrude itself. Shells of this description are called 'univalves. In others, the shell is composed of two pieces or valves, attached to each other at one point by a hinge, which is furnished with an elastic ligament that serves to open the valves, when it is not opposed by the action of the adductor muscles, whose office it is to keep the shell closed. Shells of this kind are termed bivalves. These differences in the character of the shell correspond with eiatlforniation Of Oa anintalsinhabitiag them. The• bivalve mollusca exhibit no traces of a head, and hence are termed acephalous mollusca; the univalves have a distinct head, provided with organs of the special senses, and hence, by way of distinction, some writers have termed them cephalophora (or head bearing). Many mollusca are altogether unprovided with a shell, or have only a small
calcareous plate embedded within the mantle. These are termed naked mollusca. It is worthy of notice that the young mollusc, while still in the egg, is almost always fur nished with a delicate pellucid shell, even when it is ultimately to be naked, in which case the embryonic shell is cast off soon after the animal makes its escape from the egg. For the mode of formation, etc., of the shell, see SHELL.
The movements of many of the mollusca are executed by means of a muscular struc ture concentrated in some particular part or parts of the mantle, and termed the foot. In some (the gasteropoda), the foot forms a sort of flattened disk, by the alternate contraction and expansion of different parts by which the animal can slowly crawl forwards; whilst in others (the free-moving bivalves) it is a tongue-like organ, which can be protruded between the valves, and by its sudden extension, after being previously bent upon itself, can enable its possessor (the common cockle, for example) to take considerable leaps. The foot is also the agent by means of which certain species burrow in the sand or mud, and others bore into the solid rock. Many mollusca, however, are firmly attached to a single spot, except their larval state; and as they do not require a foot., we find it either altogether undeveloped (as in the oyster), or serving to Support a glandular organ, from which filaments of silky or horny matter.(called the byssus) are secreted, which serve to attach the animal (the common mussel, for example) to rocks, stones, etc., beneath the water. Many of the subdivisions of the molluscs present modes of locomo tion altogether independent of a foot, as, for example, the biphora, which are described in the article TuNicATA; those bivalves which possess a branchial or respiratory chamber, into which water is drawn, and again expelled by muscular action, a recoil being thus produced which serves to drive the animal through the water; the pteropoda (q.v.), which are furnished with a pair of broad flattened fins (which may possibly be regarded as a modified foot) at the sides of the head, by means of which they swim with tolerable rapidity; and the cephalopoda, in which the mouth is surrounded by a number of arms, which serve not only as organs of motion, but for the capture of prey.