STRUCTURE OF MOLLUSCA.

115

bb; to this cavity, water has access by means of a wide slit, of which the edge, f, of the mantle forms one part of the border, whilst at d is seen a fringed membrane that forms another part. At c is seen the heart, which receives the blood from the gills by vb, the branchial vein, and then transmits it to the body generally; at e, far up in the spire, are the stomach and liver; at a, the anal orifice of the intestine within the branchial cavity, and at ov the oviduct, which opens in the same situation.

109. Thus it is seen that, whilst the body of an Articulated animal may be compared to that of a man in whom the apparatus of nutrition (contained in the chest and abdomen) is of the smallest possible size, but whose limbs are strong, and his movements agile,—the body of a Mollusk resembles that of a man 66 whose god is his belly," his digestive apparatus becoming enormously developed, whilst his limbs are feeble, and his movements heavy. Such varieties, in a greater or less degree, are continually presenting themselves to our notice.

110. The nervous system of the Mollusca generally consists of a single ganglion or pair of ganglia, which are placed in the head, or (when that is deficient) in the neighbourhood of the mouth; and of two or more separate ganglia, which are found in different parts of the body, and are connected with the preceding by nervous cords. The

former correspond to those contained in the head of Insects; but of the latter, one only is connected with the foot or organ of motion, the remainder having for their function to regulate the action of the gills, and to perform other movements connected with the operations of nutrition. In fig. 56 is represented one of the simpler forms of this nervous system,— that of the Pecten or Scallop-shell;

B

AA are the ganglia near the mouth, Fig. 56.-NERVOUS SYSTEM OF from which the organs of sense

PECTEN.

are supplied; вB is the ganglion connected with the gills; and c is that from which power is given to the foot. The

116

GENERAL STRUCTURE OF MOLLUSCA.-CEPHALOPODS.

two first lie wide apart, but are connected by an arched band that passes over the gullet, e. The organs of sense in the higher forms of Mollusca are more developed than those of motion. They serve to direct the animal to its food, and to warn it of danger; but there seems an absence, in all save the highest species, of that ready and acute sensibility which is so remarkable in the preceding groups; and the variety of impressions which they can receive appears to be but small. In no instance has a special organ of smell been certainly discovered; the organ of hearing is always imperfect, and frequently absent altogether; and the eyes are very often wanting. In the lower Mollusca there are no certain indications of the existence of any organs of special sense; and there is probably but a limited amount of general sensibility.

111. As the Articulata are divided into two subordinate groups, according to the presence or absence of articulated limbs or members, so may we arrange the Mollusca in two subdivisions, according to the presence or absence of a distinct head, that is, a projecting part of the body, containing the mouth or entrance to the digestive cavity, and also bearing the organs of sense which guide the animal in the discovery and selection of its food. In the higher Mollusca, there is a distinct head, furnished with eyes, and sometimes with imperfect ears; but in the lower, the entrance to the digestive cavity or stomach is buried deep among other parts, and is guarded by no other organs of sense than the tentacula or sensitive lips. These are termed acephalous, or headless Mollusca and among the lowest of them (§ 114), we meet with composite fabrics, formed by the process of multiplication by budding, which was formerly regarded as peculiar to Zoophytes.-The highest group of Mollusca, in regard to the approach of several parts of its structure to that of Vertebrated animals, is the class of CEPHALOPODA, or Cuttle-fish tribe which receives its name from the peculiar arrangement of the arms or feet around the mouth, which is the characteristic of its members (fig. 57). The common Cuttle-fish and its allies are destitute of any external protection; but they usually have a flat shel, commonly known as the cuttlefish bone, inclosed in a fold of the mantle, and lying along the back. In the Calamary, this is horny in its texture, and is sufficiently flexible to offer no resistance to the action of

:

STRUCTURE OF CEPHALOPODS AND PTEROPODS.

117

the fin-like tail, by which the animal is propelled through the water very much in the manner of a fish. The Pearly Nautilus is the only type now existing of an inferior order of

Fig. 57.-CALAMARY.

Cephalopods, which approaches the Gasteropods in many parts of its organization. The body is inclosed in the last chamber of a shell (usually spiral in form),

the cavity of which is divided by numerous transverse partitions; and such shells, the fossilized remains of very numerous forms of this group that existed in the ancient seas, constitute the nautilites, ammonites, belemnites, &c., which abound in many rocks (fig. 58). The Cuttlefish are animals of considerable activity; their mouth is furnished with a horny beak, strongly resem

Fig. 58.-AMMONITE.

bling that of the parrot; and their arms are provided with a series of very curiously constructed suckers, by the action of which they can take a very firm

hold of anything which they desire

to grasp.

112. The class of PTEROPODA, or wing-footed Mollusks, consists of but few species, and the animals which it contains are all of them of small size; but the individuals are often very numerous, whole fleets. of them being sometimes seen covering the ocean, especially in the Arctic and Antarctic regions, where they constitute one of the principal articles of food to the Whale. The general form of the body usually differs but little from that represented in fig. 59.

Fig. 59.-HYALEA.

118

STRUCTURE OF GASTEROPODS AND BIVALVES.

On either side, a little behind the head, the mantle is extended into a fin-like expansion, by the aid of which the animal can swim through the water. The hinder part of the body is usually inclosed, more or less completely, in a shell, which is commonly of extreme thinness and delicacy. The head is not furnished with long arms, to grasp the food; but it has a number of minute sucking disks, by which it can lay firm hold of whatever it attacks: whilst its powerful rasp-like tongue is set to work upon it.-The class GASTEROPODA Contains those animals which, like the Snail and Slug, crawl upon a fleshy disk on the under side of their bodies; and the number of distinct forms which it includes is very large. The greater part of them are inhabitants of the sea-shore, rivers, lakes, &c.; some have the power of swimming freely through the open sea; and the proportion of those that breathe air and live on land, is comparatively small. The general structure of the animals of this group has been already described (§ 108). Some of them form shells, whilst others are destitute of them. The shells are composed of a

single piece, or are univalve, except in one tribe ; and they have usually more or less of a spiral formation (fig. 60). The animals of this class all possess a distinct head; and this is generally furnished with eyes, as well as with tentacula. They have often a powerful masticating apparatus, and are voracious in their habits; Fig. 60.-SHELL Some of them feed upon vegetable matter, others upon animals.

OF PALUDINA.

113. The Acephalous Mollusca are divided into two groups, -those which form shells, and those which do not. The former are termed CONCHIFERA, or shell-bearing animals; and this class includes all the Mollusca that form a shell composed of two parts or valves fitted together (which shell is termed bivalve), as well as some others whose general structure is the same, but whose shell is formed in several pieces, or multivalve. The two valves of a bivalve shell (fig. 61) are connected by a hinge, where they are united by a ligament, which, by its elasticity, keeps them apart while it holds them together. This is their usual condition when the animal is alive; and in this manner the water which is required for their respiration, and also to convey their supply of food, has free access to the internal

STRUCTURE OF CONCHIFERA, OR BIVALVES.

119

parts. But when any alarm or irritation causes the animal to close its shell, it does so by means of a muscle (sometimes

single, sometimes double), which stretches across from one valve to the other, and which, by contracting, draws them together. Each valve is lined by an extended fold or lobe of the mantle. In the higher tribes of the class, these lobes are united along their edges, leaving apertures for the ingress and egress of water (which are sometimes prolonged into tubes, fig. 150), and another for the foot. But in the Oyster and its allies, which have no foot, or a very small one, the mantle-lobes are quite disunited. The accompanying diagram (fig. 62) gives a general idea of the arrangement of the organs in one of the higher acephalous Mollusca, the Mactra, which is among those having two muscles for the drawing together of the valves. The upper end, as represented in this figure, is that which is considered as the anterior end or front of the animal, being that nearest which the mouth lies; and the posterior extremity (the lowest in the figure) is that at which the intestinal canal terminates, and at which the respiratory tubes are formed. Near the anterior muscle, we find the mouth, or entrance to the stomach; it is furnished with four riband-shaped tentacula, of which one is seen in the figure; and these seem to possess peculiar sensitiveness. Near the mouth lie the anterior ganglia of the nervous system, which represent the brain of higher animals; and these are connected by long cords with the posterior ganglion, which lies near the posterior muscle. The stomach, intestines, and liver occupy the central portion of the cavity of the shell; and the intestinal tube is seen to pass backwards,