of the extinct trilobites. Limulus was formerly classified among the Crustacea, but is now considered to have its closest affinities among the Arachnida.

A mollusk is a soft-bodied animal, without internal skeleton, and without segmentation of body or of parts, covered with a moist, sensitive, contractile skin, which, like a mantle, loosely envelops the creature. In some cases the skin is naked, but generally it is protected by a calcareous covering (shell). The length of the body is less in proportion to its bulk than in other animals. The lowest class has no distinct head. The nervous system consists of three well-developed pairs of ganglia, which are principally concentrated around the entrance to the alimentary canal, forming a ring around the throat. The other ganglia are, in most cases, scattered irregularly through the body, and in such the body is unsymmetrical (Figs. 331, 332). The digestive system is greatly developed, especially the "liver," as in many aquatic animals (Figs. 242, 243). Except in the cephalopods, the muscles are attached to the skin, or shell. There is a heart of two chambers (auricle and ventricle) or three (two auricles and ventricle). As in all invertebrates, the heart is arterial. In mollusks, with rare exceptions, we find no repetition of parts along the antero-posterior axis. They are best regarded as "worms" of few segments, which are fused together and much developed. The total number of living species probably exceeds twenty thousand. The great majority are water breathers, and marine; some are fluviatile or lacustrine, and a few are terrestrial air breathers. All bivalves, and nearly all univalves, are aquatic. Each zone of depth in the sea has its particular species. The most important classes are now to be described.

CLASS I. Pelecypoda

1

Pearl Oyster

These mollusks, formerly called lamellibranchs, are all ordinary bivalves, as the oyster and clam. The shells differ from those of brachiopods in being placed on the right and left sides of the body, so that the hinge is on the back of the animal, and in being unequilateral and equivalved.25 The umbo, or beak, is the point from which the growth of the valve commences. Both brachiopods and pelecypods are headless; but in FIG. 84. the latter the mouth points the same way as the umbo, i.e., toward the anterior part. The length of the shell is measured from its anterior to its posterior margin, and its breadth from the dorsal side, where the hinge is, to the opposite, or ventral, edge. The valves are united to the animal by one muscle (as in the oyster), or two (as in the clam), and to each other by a hinge. In some species, as some fresh-water mussels, the hinge is simply an elastic liga

FIG. 85. Salt

- water

lucidus). Atlantic

(Meleagrina margari tifera); one fourth natural size. Ceylon.

ment, passing on the outside from one valve to the other just behind the beak, so that it is stretched when the valves are closed. Another is placed between the edges of the valves, so that it is squeezed as they shut, like the spring in a watch case. Such bivalves are said to be edentulous. But in the majority, as the clam and the freshwater Unio, the valves also articulate The by interlock-parts called teeth.

Mussel (Mytilus pel valves are, therefore, opened by the ligaments, and closed by the muscles.

coasts.

The shell is secreted by the mantle.

Lamellibranchs breathe by four hollow, platelike gills (whence the name), two on each side underneath the mantle (Fig. 275), the water being drawn into the cavities in the gills by the action of ciliated cells. In the higher forms, the margin of the mantle is rolled up into two tubes, or siphons, for the inhalation and exhalation of water. They feed on microscopic organisms gathered from the water by the ciliated inner surface of the mantle, the cilia producing a flow of particles toward the mouth.

A few are fixed; the oyster, e.g. habitually lying on its left valve, and the salt-water mussel hanging to the rocks by a cord of threads called "byssus"; but most have a "foot," by which they creep about. Unlike the oyster, also, the majority live in an erect position, resting on the edges of their shells. About five thousand living species are known. These are fresh-water and marine, and range from the shore to a depth of a thousand feet.

The chief characters for distinguishing lamellibranchs are the muscular impressions,26 whether one or two; the presence of a pallial sinus, which indicates the possession of siphons; the structure of the gills, and the symmetry of the valves (Fig. 296).

The following are the more important orders, classified according to gill structure:

1. Filibranchia, with two pairs of platelike gills, the filaments being V-shaped, usually two adductor muscles of which the anterior is often the smaller or may even be absent, sea mussel (Mytilus) (Fig. 85).

2. Pseudo-lamellibranchia, with gills showing vertical folds, a single, large (posterior) adductor muscle, the shell frequently inequivalve, oyster (Ostrea) (Fig. 242), scallop (Pecten), pearl oyster (Meleagrina) (Fig. 84).

3. Eulamellibranchia, with gills smooth or vertically plaited and with two adductor muscles of equal size, fresh-water mussel (Unio and Anodonta), cockle (Cardium) (Fig. 87), quahog (Venus), shipworm (Teredo), and common clam (Mya).27

CLASS 2.- Amphineura

The animals in this class were formerly placed among the Gastropoda, but are now considered to be sufficiently distinct to be grouped by themselves. They are bilaterally symmetrical, elongated mollusks, with a shell consisting of eight separate pieces, or else entirely lacking. The mantle is not divided into paired lobes as in the bivalves. Chiton, a sluggish animal with the habit of the limpet, is one of the best-known forms (Fig. 100). The shell-less members of the class are the lowest in organization of all of the mollusks.

CLASS 3.-Gastropoda

The snails are, with rare exceptions, all univalves.28 The body is coiled up in a conical shell, which is usually spiral, the whorls passing obliquely (and generally from

right to left),29 around a central axis, or "columella" (Fig. 297). When the columella is hollow (perforated), the opening in the end is called the "umbilicus." When the whorls are coiled around the axis in the same plane, we have a discoidal shell, as the Planorbis. The mouth, or "aperture," of the shell is "entire" in most vegetablefeeding snails, and notched or produced into a canal for

the siphons in the carnivorous species. The former are generally land and fresh-water forms, and the latter all marine. In some gastropods, as the river snails and most sea snails, a horny or calcareous plate (operculum) is secreted on the foot, which closes the aperture when the animal withdraws into its shell. In locomotion, the shell is carried with the apex directed backward.

The body of most gastropods is unsymmetrical, the organs not being in pairs, but single, and on one side, instead of central. The mantle is continuous around

the body, not bilobed, as in lamellibranchs. A few, as the common garden snail, have a lung; but the vast majority breathe by gills. The head is more or less distinct, and provided with two tentacles, with auditory