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the bottom of its burrow, it extends its slender siphons up the tube and out of the entrance for its food supply. Often hundreds of individuals enter the same piece of wood, which becomes thoroughly riddled within a short
taisyhelp Fig. 52.—The piddock (Zirphæa crispata), a rock-boring mollusk. Natural size,
time, and though giving no outward sign of weakness may collapse with its own weight. Incalculable damage is thus rendered to the shipping interests, and in consequence much has been done to check their ravages, but they are far from being completely overcome.
87. Other stationary species.—A large number of other species, while small and inconspicuous, are also free to
move about, but as they become larger they lose this ability either wholly or periodically. In the edible mussels (Mytilus, Fig. 53), for example, which are associated in great numbers on the rocks along our coasts, the foot early becomes long and slender and capable of reaching out a considerable distance from the shell to attach threads (byssus), which it spins, to foreign objects. These are remarkably strong, and when several have been spun it becomes a matter of much difficulty to dislodge them. After remaining anchored in one situation for a while the mussel may vol.
untarily free itself, and in a labored fashion move to some other more favorable spot where it again becomes attached, but there are numerous species, such as “fan shells” (Pinna), scallops, Anomia, and a few fresh-water forms, where the union is permanent.
Finally, in the oysters, some of the scallops, and a number of less familiar forms, the young in very early life drop down upon some foreign object to which the shell soon becomes firmly attached, and in this same spot they pass the remainder of their lives. The oyster usually falls upon the left half of its shell, which becomes deep and capacious enough to contain the body, while the smaller right valve acts as a lid. As locomotion is out of the question, the foot never develops, and the shell is held by only one adductor muscle, whose point of attachment in the oyster is indicated by a brown scar in the interior of the shell.
88. Internal organization. It is thus seen that the external features of the clam are variously modified, according to the life of the animal, but the internal organization is much more uniform. In nearly every species the food consists of floating organisms, which are driven by the palps into the mouth and on to the simple stomach, where it is subjected to the solvent action of the fluids from the liver (Fig. 51, B, 1) before entering the intestine. This latter structure is usually of considerable length, and in the active species extends down into the foot, and it is also peculiar in passing through the ventricle of the heart. Traversing the intestine the nutritive portion of the food is absorbed, and is conveyed over the body by a circulatory system more highly developed than in the higher worms. On the dorsal side of the clam, in a spacious pericardial chamber, the large heart is situated (Fig. 51, h), consisting of a median highly muscular ventricle surrounding the intestine and of two thin auricles, one on either side. From the former, two arteries with their numerous branches convey the blood to all parts of the body, where it accumulates, not in capillaries and veins, but in spaces or sinuses among the muscles and various organs, constituting a somewhat indefinite system of channels which lead to the gills and kidneys. In these organs the blood delivers up the waste which it has accumulated on its journey, and absorbing a supply of oxygen, it flows into the great auricles, which in turn pass it into the ventricle to circulate once more throughout the body.
The excretory apparatus, consisting usually of two kidneys, of which one may degenerate in many snails, bears a close resemblance to that of the annelids. In the clam, for instance, each consists of a bent tube symmetrically arranged on each side of the body (Fig. 51, B, k), and the inner ends (a), corresponding to the ciliated funnel of the annelid kidney, open into the pericardial cavity. The walls are continually active in extracting wastes from the blood supplied to them, and these, together with the substances swept out from the pericardial cavity, traverse the tube and are carried to the exterior. In other mollusks the kidney may be more compact, or greatly elongated, or otherwise peculiar, but in reality they bear a close resemblance to those of the clam.
89. Nervous system.—The nervous system, like the excretory, differs considerably in different mollusks, yet the resemblances are fairly close throughout. In the clam the cerebral ganglia corresponding to the “brain” in annelids is located at either side, or above the mouth, and from it several nerves arise, the larger passing downward to two pedal ganglia (p) embedded in the foot and to the visceral ganglia (v) far back in the body (Fig. 51, B). These nerve centers continually send out impulses which regulate the various activities of the body and also receive impressions from without. These come chiefly through the sense of touch, for in the clams the other senses are usually either feebly developed or altogether absent.
90. Development.—In the mollusca new individuals always arise from eggs, which are commonly deposited in the water and there undergo development. In the fresh-water clams the reproductive organ is usually situated in the foot (Fig. 51), while in the oyster and similar inactive species it is attached to the large adductor muscle. In these latter, and in many other marine forms, the eggs are shed directly into the sea, where they are left to undergo their development buffeted by winds and waves and subject to the attack of numerous enemies. Under such circumstances the chances of survival are slight, and for this reason eggs are laid in vast numbers, which have been variously estimated for the oyster, for example, from two to forty million. Development proceeds at first much as in the sponge, but soon the shell, foot, gills, and various other molluscan structures put in an appearance, and the few surviving young which have been free-swimming now settle down in some favorable spot, and attach themselves or burrow according to their habit.
91. Life history of fresh-water clams.—The life history of our common fresh-water clams is perhaps one of the most remarkable known among mollusks. The parent stores the eggs, as soon as they are laid, in the outer gill plate, and there, well protected, they undergo the first stages of their development, which results in the formation of minute young enclosed in a bivalve shell beset with teeth. These are often readily obtained, sometimes as they are escaping from the parent, and when examined under the microscope are seen to rapidly open and close their shells in a snapping fashion when in the least disturbed. In a state of nature this latter movement may result in attaching the young to the fins or gills of some passing fish, which is necessary to its further development. Within a short time it becomes completely embedded in the flesh of its host, from which, as a parasite, it draws its nourishment, and during the next few weeks undergoes a wonderful series of transformations resulting in a small mussel, which breaks its way
through the thin skin of the fish and drops to the bottom. in 92. The gasteropods.—The gasteropods, including snails,
slugs, limpets, and a host of related forms, fully twenty thousand different species in all, are found in most of our fresh-water streams and lakes and in moist situations on land, while great numbers live along the seashore and at various depths in the ocean, even down as far as three miles. Examining any of them carefully we find many of the same organs as in the clams, but curiously changed and adapted for a very different and usually active life. In our common land snails (Fig. 54), which we may well examine before passing on to a general survey of the group, the first