CHAPTER XIV. OF THE MOLLUSCA IN GENERAL. 862. QUITTING now the Articulated series,-which we have seen to terminate in very simple forms of structure, that lead us towards the Zoophytes, we return to a higher point in the Animal scale, to descend in like manner through the Molluscous series. Looking only at the general complexity of structure exhibited by these groups of Animals respectively, we might have some difficulty in deciding which should rank the highest; since in the Articulata we observe one set of organs far more highly organised than in any of the Mollusca; whilst the converse holds good as to another set of organs in the Mollusca. But when we look at the respective characters of these organs, we cannot reasonably hesitate longer. It is the organs of animal life that are so highly developed in the ARTICULATA; whilst it is in the development of the organs of nutritive or vegetative life only, that they are surpassed by the MOLLUSCA. In the Animal scale, therefore, we may justly regard the Articulata as ranking on the whole above the Mollusca. To the consideration of the latter group we shall now proceed. 863. The range of animal forms comprehended in the subkingdom MOLLUSCA is so great, that it would be difficult to include them by any character common to all. We encounter but few traces of the circular disposition of organs around the mouth, which is characteristic of the Radiated tribes; and we seldom meet with any thing that even approaches to the elongation of the body, still less to the division into segments, which has been noticed among the Articulata. This will be comprehended, when it is borne in mind that the body of the Mollusca is almost entirely occupied by the organs of nutrition; and that the organs of sensation and locomotion are entirely subservient to GENERAL CHARACTERS OF MOLLUSCA. 327 the supply of these. We find in the lowest tribes of this group, living beings which are fixed to one spot during all but the earliest period of their lives; and which scarcely possess within themselves so much power of movement, as that enjoyed by the individual polypes in a mass of Coral; and yet these exhibit a complex and powerful digestive apparatus, a regular circulation of blood, and an active respiration. But we nowhere find, throughout the whole animal kingdom, that the conformation of these organs governs the shape of the body; they rather adapt themselves to the type which predominates in its structure, and which is principally manifested in the disposition of the locomotive organs. Thus, the stomach of the Star-fish sends a prolongation into each ray; whilst in the Articulata, on the other hand, we find the digestive cavity prolonged into a tube, in accordance with the form which the body there possesses. 864. Thus we see that, in regard to external shape and arrangement, the apparatus of Organic life has no definite plan of its own; and in the Mollusca there is an absence of any general type, to which it may be made conformable. Hence the shape of the body varies extremely in those classes, in which it is entirely or principally composed of these organs; and no general character can be given, which shall apply to all, or even a large part, of the animals composing them. There is often an entire want of every kind of symmetry; that equality of the two sides, which is peculiarly characteristic of the higher animals, being deficient; as well as the radial arrangement of parts seen in the lower. But this is only the case, where there is no development of a head; that is, of a prominent part on which the mouth is situated, and which also bears the organs of sensation, if any exist. In the higher Mollusca, which possess not only sensory tentacula, but eyes, and even organs of smell and hearing, we find these disposed in a symmetrical manner; so that the head (where it exists), or the part peculiarly concerned in animal life, presents a bi-lateral equality of parts, even where the remainder of the body wants it. In the more active species among the higher classes, we find this bi-lateral symmetry éxtending in many instances through the whole body; evidently 328 GENERAL CHARACTERS OF MOLLUSCA. bearing a pretty close relation with the degree of locomotive power. It is most evident and complete in the CEPHALOPODA ; many of the animals in which class are adapted to lead the life of Fishes, and resemble them in the general form of the body and in the structure of many individual organs. 865. As a group, however, the MOLLUSCA are to be characterised rather by the absence, than by the possession, of a definite form; and there is a corresponding absence of any regular organs of support, by which such a form could be maintained. The name they have received designates them as soft animals; and this they are pre-eminently. The Shell, where it exists, is to be regarded rather in the light of an appendage designed for the mere protection of the body, and deriving its shape from it; than as a skeleton, giving attachment to muscles, and regulating the form of the whole structure. Where t'e body is entirely inclosed within it, as in the lower bivalve Mollusca, no locomotive powers whatever, except such as depend on the passage of water through the respiratory tubes, are enjoyed by the animal. It is only where the body is uncovered by a shell, or a portion of the body can be projected beyond it, that any active movements can be executed; and the muscles concerned in the performance of these do not make the shell a fixed point, as is done by those of Articulated or Vertebrated animals in regard to their skeletons, but are entirely unconnected with it. 866. Hence we see that the shell of a Mollusk is, when considered in reference to its functions, a very different organ from that of a Crustaceous animal, although formed in somewhat the same manner. Its frequent absence might of itself lead us to suspect its want of importance to the living structure. In one whole class of Mollusca it is entirely deficient; and in three others it is frequently absent. In only one it is universally present. When speaking of the anatomical conformation of the body, therefore, we may leave the shell pretty much out of consideration. Before the animals which produce them had been properly studied, Naturalists founded their classification of Mollusca upon the shells only; and the greatest confusion thus resulted. Shells GENERAL CHARACTERS OF MOLLUSCA. 329 of very similar aspect are often produced by animals extremely unlike each other,* and living in different conditions-as, for instance, fresh and sea water. And shells of very dissimilar character in the eye of the mere Conchologist, often belong to animals closely allied. In fact, the form of the shell taken. alone is a character as purely artificial, as the number of stamens and pistils in a flower; and will lead to a classification as far removed from a natural plan. But when the principal divisions have been formed upon other grounds, the conformation of the shell will often afford valuable subordinate characters; and the Naturalist seeks to employ these as much as he safely can, on account of the facility with which he can apply them to the study of those fossil remains, from which all traces of the animal itself have disappeared. The softness of the entire body of the Mollusca prevents us from recognising its form and structure after death, in any other way than by the shell; but upon this, it must be remembered, entire reliance cannot be placed, since it is liable to great variation, in accordance with the circumstances of the individual, whilst it is by no means certain that there are constant differences in its form in distinct species. 867. The only tribe of Mollusca which presents anything that corresponds to the internal skeleton of the Vertebrata, is that of the Cuttle-fish. There is in their bodies a cartilage, partly inclosing the nervous centres in the head, and sending prolongations along the back, for the protection of the large nervous cords which traverse it, and for the attachment of the muscles by which it is moved. These last are especially developed, where the body is spread out into fin-like processes, resembling those of Fishes, by the stroke of which active movemerts are produced (Fig. 540). This skeleton is almost ast highly organised, as are the lowest forms of that internal skeleton which is characteristic of Vertebrata (§ 585). 868. In all the Mollusca, the soft body of the animal is The impossibility of founding a correct classification upon the characters afforded by the shell only, is made at once apparent by comparing the shell of a Serpula (Fig. 528), with that of a Vermetus (Fig. 583). Between these two shells, there is no essential difference; yet the animals which form them belong, not merely to different families, but to different sub-kingdoms. 330 MANTLE AND SHELL OF MOLLUSCA., inclosed in a spongy elastic skin, with which muscular fibres are interwoven; this is termed the mantle. This envelope is frequently not applied closely to the surface of the organs contained in it; especially among the lower classes, in which the space thus left constitutes a respiratory chamber. It has apertures for the admission and exit of the surrounding water, to effect the aeration of the blood; and, when the mouth is not capable of being projected beyond it, the same current furnishes the supply of food. These apertures are sometimes extended, for particular purposes, into proboscis-like tubes (Fig. 597). Where the head is capable of being protruded, there is usually an opening in the mantle for the purpose; and another for the foot, where it exists as a separate organ. 869. The mantle is chiefly interesting, as being the portion of the body alone concerned in the formation of the shell. Sometimes this envelope is secreted from nearly the whole surface; sometimes from only a small part of it. As the same general statements in regard to the constitution of this body will apply to all the Mollusca, its origin and essential characters may advantageously be described, previously to the more detailed account, which will be given of the several classes, in which its leading peculiarities of form will be noticed as they occur. Shells are formed, like bones, of a combination of earthy and animal matter. The former consists entirely of carbonate of lime, which is usually deposited in a crystalline condition. The latter is composed of layers of membrane, alternating with the mineral matter; and of cells inclosing it. The cellular structure, which seems to be of the nature of Epithelium (ANIM. PHYSIOL. § 39), is generally found upon the surface. If the carbonate of lime be removed by the slow action of a weak acid, the animal matter will remain, sometimes in the form of a continuous membrane, but sometimes only as a flaky mass, easily shaken into separate portions. The shell is most solid and massive in those species which lead an inactive life; and it attains greater weight in the Conchifera, in which every species forms a stony covering, than in the Gasteropoda, whose powers of locomotion are somewhat greater, and in which the shell is often absent, |