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by the retreating tide. They are usually highly colored with yellow, green, violet, or various shades of red, and are so twisted into tangled masses that the different parts of the body are indistinguishable. As the animal crawls about, a long threadlike appendage, the proboscis, is frequently shot out from its sheath at the forward end of the body and appears to be used as a blind man uses his stick. At other times, when small worms and other animals are
encountered, the proboscis is shot out farther and with greater force, impaling the victim on a sharp terminal spine (Fig. 50). The food is now borne to the mouth, located near the base of the proboscis, is passed into the digestive tract, traversing the entire length of the body, and is farther operated on by systems of organs too complex to be considered here.
84. General characters.—For very many years the mollusks—that is, the clams, snails, cuttlefishes, and their allies —have been favorite objects of study largely because of the durability, grace, and coloration of the shell. The latter may be univalve, consisting of one piece, as in the snails, or bivalve, as in the clams and mussels, and may possess almost every conceivable shape, and vary in size from a grain of rice to those of the giant clam (Tridacna) of the East Indian seas, which sometimes 'weighs five hundred pounds. These external differences are but the expression of many internal modifications, which, while adapting these animals for different modes of life, are yet not sufficient to disguise a more fundamental resemblance which exists throughout the group. In some respects the mollusks show a close resemblance to the annelid worms, but, on the other hand, the body is usually more thick-set and totally devoid of any signs of segmentation. In every case the skin is soft and slimy, demanding moist haunts and usually the protection of a shell, and the body is modified along one surface to form a foot or creeping disk which serves in locomotion. The internal organization is somewhat uniform, and will admit of a general description later on. Mollusks are divided into three classes, viz.: The Lamellibranchs, embracing the clams; the Gasteropods, or snails; and the Cephalopods, or cuttlefishes, squids, and related forms.
85. Lamellibranchs (clams and mussels).—Numerous representatives of this class, such as the clams and mussels, occur along our seacoasts or are plentifully distributed in the fresh-water streams and lakes. They are distinguished from other mollusks by a greatly compressed body, which is enclosed in a shell consisting of two pieces or valves locked together by a hinge along the dorsal surface. Raising one of these valves, the main part of the body may be seen to occupy almost completely the upper (dorsal) part of the shell (Fig. 51), and to be continued below into the muscular hatchet-shaped foot 9/C), which aids the clam in plowing its way through the sand or mud in which it lives. Arising on each side of the back of the animal and extending its entire length is a great fold of skin, which completely lines the inner surface of the .corresponding valve of the shell. These are the twp^man>ie lobes (m) instrumental in the formation of the^shell, and enclosing between them a space containing the foot and a number of other important structures, the most conspicuous of which are the gills (g), consisting of two broad, thin plates attached along the sides of the animal and hanging freely into the space (mantle cavityj,4>etveen <fh'e mantle and the foot. Owing to this lamella-like character of the branchia or gills the class derives its name, lamellibranch. To illustrate the relations of these various organs to one another the clam has been compared to a book, in which the shells are represented by the cover, the fly-leaves by the mantle lobes, the first two and last two pages by the gills, and the remaining leaves by the foot. In the clams, however, the halves of the mantle, like the halves of the shell, are curved, and thus enclose a space, the mantle cavity, which is partly filled by the gills and foot.
Unlike the other mollusks which usually lead active and more aggressive lives, the clams show scarcely a sign of a head and tentacles, and other sense organs are likewise absent from this region. The mouth also lacks definite organs of mastication, and as devices for catching and holding food are not developed, the food is brought to the
Fig. 51.—Anatomy of fresh-water clam. A, right valve of shell removed; B, dissection to show internal organs, a, external opening of kidney; a.a., the anterior muscle for closing the shell; 6, opening of reproductive kidney; c, brain; ft,, foot; g, gill; h, heart; i, intestine; k, kidney; I, liver; m, mantle (upper fig.), mouth (lower fig.); p, palp (upper fig.), foot nerves (lower fig.); p.a,, hinder muscle for closing the shell; s, space through which the water passes on leaving the body ; stt stomach ; v, nerves supplying viscera.
Between the halves of the shell in the hinge region is a horny pad that acts like a spring, and without any muscular effort on the part of the clam keeps the shells open. These are also united by two great adductor muscles, located at opposite ends of the animal (Fig. 51, A.a.a.,p.a.), which in times of disturbance contract and ifir/niy close the shell. Upon their relaxation the shelbjppeipf the clam extends its foot, and plows its way leisurely through the mud, or remains buried, leaving only the hinder portion of its gaping shell exposed. Through this opening a current of water is continually passing in and out, owing to the action of the cilia covering the gills, and by placing a little carmine or coloring matter in the ingoing stream we may trace its course through the body. Passing in between the mantle and the foot it travels on toward the head, giving off small side streams which are continually made to enter minute openings in the gills, whence they are conducted through tubes in each gill up to a large canal at its base, where it is carried backward to the exterior. In this process oxygen gas is supplied to the number of blood-vessels traversing the gills, and at the same time considerable quantities of minute organisms and organic debris are hurried forward toward the head, where they encounter the whirlpools made by the cilia on the lips and are rapidly whisked down into the mouth and swallowed.
86. Rock- and wood-boring clams.—Other similar forms are rendered even more secure through their ability to bore in solid rock. In the common Piddock, for example (Fig. 52), the shell is beset with teeth like a rasp, which gradually enlarge the cavity as the animal grows, until it becomes a prisoner with no means of communication with the exterior save the small opening through which the siphons project. This is also the case with the Teredo, frequently called the shipworm, which swims about for some time during early life and then, about the size of a small pinhead, settles down upon the timbers of wharves or unsheathed ships, into which it rapidly tunnels. Throughout life its excavation is extended sometimes to a distance of two to three feet, and imprisoned yet safe at