TRIDACNE.CARDIACE.E. 411 whole regiment is scarcely an exaggeration, as the flesh of these animals is commonly used as food where they abound, and is by no means unpalatable. The remarkable difference of the shell of this animal at different epochs of its growth, has given rise to the formation of many species which have no real existence. It is only when immature, and when the shell is comparatively light, that the animal is attached by a byssus. This cord, however, seems rather to be a musculo-tendinous prolongation of the foot itself, than a fibrous tissue secreted by it like the byssus of the Pinna; it is so tough as to require to be chopped with a hatchet, in order that the shell may be detached. As the animal approaches adult age, however, and has by successive layers very much increased the weight of the shell, the byssus, being no longer required to secure it from injury, disappears, and the groove in the shell is filled up with a solid deposit. When thus free, it is said to be taken with a long pole, which is introduced between the valves when open; the animal immediately closes the valves upon it, and does not quit its hold until it is landed. ORDER VIII.-CARDIACEÆ. 953. This Order, including the Cockles and their allies, contains several genera, which, in the smallness and delicacy of many of their shells, and in the comparative activity of the animals that form and inhabit them, offer a remarkable contrast to those of the previous group. The shells are all equivalve, or nearly so; they are furnished with a regularly-toothed hinge, Fro. 597.-TELINA. often of great complexity and pre important organ to the animals, most of which use it in the excavation of hollows in the sand or mud of the shores on which they reside, as well as for progression. The respiratory orifices are 412 CARDIACEÆ;-CARDIUM, OR COCKLE. usually prolonged into tubes (Fig. 597); which can, however, be drawn within the shell by means of a retractor muscle. 954. In the Cardium, or Cockle, the tubes or siphons are shorter than in most of the other genera; indeed they are sometimes reduced to mere openings; and scarcely any vestige of a retractor muscle exists. The foot is very large, and is capable of being bent at an acute angle, and then suddenly straightened; so as to enable the animal to move from place to place by a succession of leaps. But it is only occasionally, that it serves this purpose. The chief use of the organ is as a boring instrument, by which the animal may penetrate the sand or mud, below the surface of which it is usually found. A very curious provision exists for adapting it to this object. As usually seen, the foot, when extended, tapers gradually to a point; and, as its diameter is at its largest point much less than the breadth of the shell, it is not apparent by what means the hole that is excavated is made sufficiently large for the reception of the latter. This is accomplished, however, by the distension of the foot with water, through a tube which opens just within the mouth; and thus the size of the borer becomes so nearly equal to that of the shell, that (its solid point first entering the sand) it is enabled, by rotatory motions often repeated, to excavate a burrow large enough to receive the animal with its shell. The Cardia are found in all known seas; and in some they abound so much, that they become very important articles of food to Man, as well as to marine animals. Mr. Kirby mentions that, on the North East coast of Norfolk, an alteration in the sands has taken place, which has caused a great diminution of late years in the number of boring bivalves; and that the quantity of Soles and other Flat Fish frequenting the coast, of which they form the principal food, has consequently much decreased also. 955. Nearly allied to the Cardium are a considerable number of genera of great interest to the Conchologist; many of them being remarkable for the beauty of their shells, or for the curious situations in which they live. The greater number of them inhabit sand or mud; but there are several which bore into rocks; and a few that burrow in masses of coral. The CARDIACEÆ.—SOLENIDÆ; RAZOR-SHELL. 413 means by which they make their excavations, are not understood. The resemblance both in the shell and the animal, among these numerous genera, is often so strong, as to produce a difficulty in their classification, as well as to render it unnecessary to enter here into details respecting them. It will be sufficient to name the genera, Venus, Mactra, Tellina, Donax, Lucina, Petricola, and Venerupis, as including the greatest proportion of the group; these being names with which even the ordinary Shell-Collector must soon become familiar, on account of the large proportion that the bivalves of this Order bear to others, on almost every coast. ORDER IX.-SOLENIDE. 956. The Mollusks of this Order are distinguished from those of the preceding, by the wide gape of their shells at the posterior extremity, and by the length of the respiratory tubes. Their habit is to burrow much more deeply; and their foot (which is of small size) is made to project rather from the anterior extremity, than from the middle of the body; so that the form of the whole is more cylindrical than we have yet seen it, though not so much so as in the succeeding Order. The Solen, or Razorshell, is a well-known example of this group. It has an elongated shell, of which the hinge is furnished with distinct teeth, and the ligament is altogether external. The animal burrows in the sand, into which it sinks rapidly on the approach of danger. It seldom or never quits its hole; and its movements are nearly limited, therefore, to an ascent or descent in it. This it accomplishes by means of its foot, which it elongates and attenuates into a point, when it wishes to bore; contracting it into a rounded form, so as to fix it by its enlargement within the hole, when it desires to rise. The animal is sought for by fishermen on some coasts, as a bait for certain fish. Its burrow is often recognised by the little jet of water which the animal throws out, when alarmed by the shaking of the sand occasioned by the motion of the fisherman above. When the tide is low, 414 SOLEN; MYA; BYSSOMIA. the holes are often seen in considerable numbers; and this is also the time when the animal may be most easily procured. The fisherman throws a little salt upon the hole, which induces the animal to ascend, according to some, by leading it to the belief that the tide had returned,—and, according to others, by an irritating effect of which it desires to get rid. To seize it when it makes its appearance, some address and quickness are required; for it speedily returns to its burrow; and, if entrapped, its struggles are sufficiently powerful to cause injury by the sharp edges of the shells. If it re-enter its hole, fresh pinches of salt no longer produce the same effect; the animal having either learned by experience that they do not indicate the return of the sea above it, or deeming it better to submit to the irritation than to expose itself to capture. The fisherman then has recourse to a long iron crook, which he sinks pretty deeply; and, drawing it out obliquely, carries away the sand, and the Solen contained in it. If he should fail in this attempt, he knows that to try again would be useless; since the animal instantly burrows rapidly down to such a distance, as to render pursuit of this kind useless. 957. This group is connected with the preceding by the Mya, Lutraria, and other genera, which are common on our coasts; and which burrow into sand or mud. In the Mya and its allies, the two respiratory siphons are united into a single tube, which is of fleshy consistence, and which is covered by an epidermis prolonged over it from the shell. One genus, the Byssomia, is remarkable for possessing a byssus at the base of its small foot. The shells of this Order are usually covered with a thick epidermis, or horny skin. ORDER X.-INCLUSA. 958. The last Order is one of the most interesting of the whole group, as regards the habits of the animals composing it, and the curious varieties of structure which they present. The peculiar disposition of the mantle, which has been noticed as characterising the group ($936), has an evident relation with the habits of the animals composing it; which have the power of forming their habitations, not only in sand and mud, but by excavating wood and rock. No general description can be given of the shells of this group; for although they often bear some resemblance to those of other bivalves, they frequently depart from them so completely, as to leave their character in doubt, until the structure of the animal producing them has been examined. The tubes cannot in any instance be drawn within the valves; and these frequently cover but a small part of the whole surface. We then often find a kind of supplementary shell, formed by a calcareous exudation, lining the hollow which the animal has pierced; and this sometimes involves the original shell, so that the latter is entirely lost in it. 959. As connecting this Order with the preceding, and as departing least widely from the general type of Bivalves, we may first mention the Pholas; this has a shell formed of two principal valves, which leave a considerable space between them. at each end when they are closed; and of supernumerary pieces, the number and position of which vary considerably. The foot issues at the opening through the larger end; and the respiratory siphons, which are very long and extensible, pass out by the other. Some Pholades form their cells in mud or clay; but many in rocks, and others in wood. It is evident that a fleshy foot can be of little use in the excavation of a stony mass; and the organ here appears to serve a different purpose. The boring operation seems to be performed (in many cases at least) by the shell itself, which has a rasp-like surface, and which is renewed by vital action as fast as it is worn down. In order to make the valves rotate backwards and forwards (like a surgeon's trephine), the foot is affixed to the bottom or end of the hole, and becomes a fixed point from which the muscles can act. They seem to commence this operation almost as soon as they quit the egg; the young beginning to bore the rock on which they are cast, and enlarging their cell, which they never voluntarily quit, in accordance with their own increase in size. They possess a very curious means of freeing the tube from the raspings of the rock produced by their penetration. The siphon |