CLASS IV. CONCHIFERA.
(Bivalve Mollusks.)
It needs but a glance at a Cockle or an Oyster, to perceive that it is an animal lower in the scale of existence than a Snail or a Periwinkle. The absence of anything like a head, of any distinct mouth, of jaws, or tongue, or other apparatus for selecting and seizing food, as well as of the organs of sensation, together with the limited power of locomotion, proves its inferiority; and this position is fully borne out by an investigation of its anatomy.
The first character that strikes us on looking at one of these Mollusks is, that it is enclosed, more or less completely, within a shell composed of two pieces, called valves, which commonly bear a close resemblance to each other. They are united at one portion of their outline by a hinge, which allows them to separate to a certain extent, while they can be, during the life of the animal, brought together with accuracy, and held in this position with great force.
If, now, we open the shelly valves, and examine the interior, of the common Scallop (Pecten), for instance, we see that each is lined with a delicate membrane, the edges of which meet in the same manner as those of the valves. These edges are slightly thickened, studded with coloured glands, and fringed with rows of close-set, thread-like, contractile tentacles. Proceeding now to separate these membranous leaves, which together compose the mantle, we find two pairs of other leaves, composed of radiating fibres of exquisite structure, attached to the body in one part, but elsewhere floating freely, so as to allow the surrounding water to bathe every part of their surface. These organs are the gills, and, on being examined with the microscope, show how beautifully their exquisite structure is contrived for the maintenance of a constant current of water over them. Each of the four leaves is then seen to consist of a vast number of straight slender transparent filaments, evidently tubular, arranged side by side, so that 1,500 of them would be contained within the length of an inch. Strictly, however, these are but one filament, excessively long, bent upon itself again and again, at both the free and the attached end of the gill-leaf, throughout its whole extent. This repeated filament is armed on each of two opposite sides with a line of vibrating cilia, the two lines moving in contrary directions; by the action of which a current of water is made continually to flow up and down each of these delicate filaments; so that the blood which circulates in their interior (for they are doubtless blood-vessels) is continually exposed throughout this its long and tortuous course to the action of oxygen.
Like all organic functions, the action of these cilia is not under the will of the animal. It is said that if, during life, a small portion of the gills be cut off, the motion of the cilia will convey the fragment swiftly away, with a smooth easy motion, through the surrounding fluid in a definite direction. It does not even cease with the life of the animal. A specimen which I examined had been dead at least fifteen hours, yet when I placed the torn fragments of the branchiæ, one after another, beneath the microscope, the energy of the ciliary action, as the wave flowed with uniform regularity up one side and down the other of every filament, filled me with astonishment. Even the next morning, twenty-six hours after death, when the tissues of the filaments were partially dissolved, the ciliary motion was still going on, on portions that preserved their integrity.
In a sort of hood formed by the union of the gill-leaves at their basal part, is placed the entrance of the stomach; a simple orifice without jaws, teeth, or tongue, but bordered by four thin, membranous lips.
The force with which the valves resist our attempts to open them, during the life of the animal, depends on the presence of a large and powerful muscle, which is very apparent when they are opened, as it occupies a considerable portion of the interior, stretching directly from one valve to the other, and inserted by a broad base into each. By the contraction of the fibres which compose this muscle, the valves are strongly pulled together, and it is by cutting across this with an inserted knife that an oyster is commonly opened. When death ensues, the valves open spontaneously from another cause; muscular contraction then ceases, and the relaxed fibres can no longer resist the expansive force of a dense and highly elastic substance placed at the back of the shell close to the hinge, and known as the ligament.
Around this great muscle are grouped the stomach, liver, intestine and other vital organs; while in the neighbourhood of the mouth there is usually found a fleshy organ capable of being protruded beyond the limits of the shell, and of assuming various forms, suited to the different offices which it has to perform. This organ, which is small in the Pecten, is, in other genera, as the common Cockle, for example, developed to a very large size; while in others, as the Oyster, no trace of its presence is to be found. It is commonly known as the tongue or foot.
The current which plays over the gills subserves also another purpose, not less necessary to life. I have already said that the Bivalve Mollusca are unprovided with any organs for pursuing or seizing prey; and yet their food is largely animal. It consists principally of those minute creatures belonging to the Class Infusoria which, invisible to the unassisted eye, swarm in innumerable millions in the waters both of the sea and of rivers. The currents which pour across the gills are crowded with these minute creatures, and the parts that surround the mouth lying in their course, and being themselves also clothed with vibratory cilia, a portion of the stream is drawn into the gullet and passes into the stomach, carrying with it the tiny prey, on which the Mollusk fattens, without any exertion of its own, for the ciliary action is doubtless, to a considerable extent, involuntary.
As far as is at present known all the members of this class are hermaphrodite; that is, the reproductive organization, instead of being assigned to two sexes, is perfect in every individual animal. In the Oyster, the ovary may be seen through the mantle, resting, as a whitish mass of considerable size, upon the muscle. It occupies the whole upper part of the animal, and creeps down along the sides and lower parts, being filled, at the time of reproduction, with a milky fluid containing multitudes of small granules of a whitish colour. These are the eggs; and in many of the family they are not at the time of their exclusion abandoned at once, but are deposited between the membranes of the gill-leaves, where they undergo a kind of incubation. In some, the shell is developed in the egg before it quits this receptacle. This fostering of the eggs seems to be analogous to the gestation of the eggs in the Crustacea and the Pipe-fishes.[1]
I have hitherto spoken of but a single adductor muscle, but in a large number of species there is a second, placed near the front part of the animal. This variation naturalists have used to divide the Class into two Orders, denominated Dimyaria and Monomyaria, or respectively Double and Single-muscled Bivalves. These characters can be determined at a glance by looking at a single valve of any shell; for the place of attachment of the adductor muscle is marked by a distinct sunken impression in the interior of each valve. From this circumstance also, these impressions, by the differences in their form, position, and dimensions, afford excellent characters for the discrimination of genera.
The accompanying figure, representing the interior of one of the valves of a common shell, will serve to illustrate the appearance of the muscular impressions, as well as of some other parts that are commonly mentioned in technical description. The oval mark on the left-hand side of the figure is the front muscular impression; the pear-shaped mark on the opposite side is the hinder one: the bending line which connects them is 
VALVE OF CYTHERÆA.
called the pallial impression, or that which marks the attachment of the mantle. On the upper side is seen the hinge, an apparatus of shelly teeth alternating with cavities, which fit, with the most accurate precision, corresponding cavities and teeth in the opposite valve. The very summit of the shell, which often, as in this instance, projects into a blunt point, bending towards one side, is termed the beak or umbo; beneath this, on the anterior side, or that towards which it turns, there is a depressed space, often very conspicuous, termed the lunule; while on the hinder side of the umbo is spread the dense elastic leathery ligament. The lunule, umbo, and ligament, constitute the dorsal or superior border, the lower side (in the figure) is the ventral or inferior border; a line drawn from one to the other of these measures the length of the shell, the width being, of course, at right angles to this, while the thickness is determined by a line from the centre of one valve to that of the opposite.
- ↑ Penny Cyclop, vii. 432.