LITMIR.BIZ

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is, it has been generally adopted, and I shall not hesitate to make use of it.

      As the great Vertebrate Division includes the four distinct Classes of Beasts, Birds, Reptiles and Fishes, so does the great Division of Mollusca contain six Classes, distinguished by characters which I shall presently enumerate. I must, however ​ever, first indicate those which they possess in common, and by which they are naturally grouped together.

      The nervous system demands our first attention. Instead of a great mass of nervous substance accumulated in one place, and a lengthened spinal cord proceeding from it, giving out threads to all parts of the body, as in the Vertebrata, we find the nervous centres numerous, unsymmetrical and disposed in various parts of the system, no one having so decided a predominance over the others in bulk, as to merit the appellation of a brain. There is, however, one mass larger than the rest, which is always placed either above the gullet (œsophagus), or encircling it, in the form of a thickened ring; and from this the nerves that supply the organs of sense invariably originate. This mass, or ganglion, must undoubtedly be regarded as the representative of the brain; for in the most highly organized animals of the Division, the Squids and Cuttles (Cephalopoda), this encirling mass is enclosed and defended by a case of cartilage, the lingering rudiment of a bony skull.

      The accompanying engraving, which is copied from Professor Grant's "Outlines of Comparative Anatomy," will give the reader an idea of the system of nerves and ganglia, with some of the other organs, as they appear in Bulla lignaria, a large and handsome shelled Mollusk found on the British coasts.

      In the above figure the chief ganglion forms a ring, (marked ee,); anterior to this there is a small ganglion, not seen, because situated below the bulb of the gullet (a), just before the insertions of its diverging muscular bands (c), and behind the ​salivary glands (b). The brain-ring (e) has on each side a large three-lobed ganglion (f), whence numerous nerves pass to the surrounding parts, and two long branches (h) extend backwards along the sides of the abdomen, to two ganglia (i,i), placed above the muscular foot. NERVES OF BULLA Behind these are two sympathetic ganglia, (k, k), which send threads to the digestive system, the ovary (o), the oviduct (p), the uterine sac (q) , the vulva (m), and the urinary organs (n). This may be considered as a fair average sample of the nervous system in the Mollusca, being selected from a Class presenting neither the highest, nor the lowest forms of organization.

The nervous centres are, for the most part, grouped without regard to symmetry, those of one side not corresponding to those of the other; and this irregularity is characteristic of the whole Division, not only in the nerves, but in the other organs of the body. Some zoologists have derived from this peculiarity, a name for the Division, ​sufficiently expressive, though too uncouth for general adoption, that of Heterogangliata.^[1]

      All the senses common to the higher animals are found in the Mollusca, though some are, doubtless, wanting in the humbler Classes of the Division. In the Cephalopoda, the organs of sight and hearing are distinct and well developed, and Professor Owen is of opinion that the Nautilus, an animal of this Class, possesses an organ of "passive smell." The Gasteropoda are almost invariably furnished with eyes; and, according to M. Siebold and other zoologists, with ears also, a pair of round capsules, placed near the bases of the tentacles, and enclosing one or more crystalline globules, called otolites. Some of the Conchifera are furnished with numerous eyes, placed among the tentacles, examples of which are found in the Clams and Scallops (Pecten) of our own shores. I scarcely know a more beautiful sight of the kind, than is presented by the edges of the mantle in one of our Scallops. If you ever have an opportunity of procuring a living specimen, which is not difficult to find at low water, on most of our rocky shores, place it in a glass of sea-water, and watch its movements. Soon the beautiful painted shells will begin to open, and the fleshy mantle will be seen to occupy the interval, like a narrow veil extending perpendicularly from each shell. The edge of each of these veils will now be seen, if you examine it with a pocket lens, to be fringed with long white threads, which are the tentacles, or organs of touch; and amongst them lie scattered a number of minute points, having the most brilliant lustre, and bearing a close resemblance to tiny gems. Indeed, the ​mantle has been aptly compared to one of those pincushions which are frequently made between pairs of these very shells, the eyes representing a double row of diamond-headed pins, set round the middle. It is observable that the Bivalves, which are thus profusely furnished with eyes, are also

      THE GREAT SCALLOP.

      endowed with the faculty of precise and vigorous motion. It does not appear clear that any of this Class possesses a distinct sense of hearing.

      The faculty of taste is plausibly conjectured, rather than proved, to belong to the Mollusca. "It seems necessary," says Dr. Johnston, "to suppose the existence of this sense in all Mollusca, for they select particular articles of food in preference to others; and we know no other sense which is fitted to regulate the choice." The organs appropriated to this faculty are probably the margins and internal surface of the mouth, and the tentacles ​which in some species are placed close to this orifice.

      Every one who has touched a crawling Slug or Snail, must have had a practical proof of the delicacy of its sense of touch. The whole surface of the body, invested with a soft, flexible, and mucous skin, contracts on the slightest contact with any unexpected substance, and is, doubtless, an extended organ of feeling, probably much more sensitive than the naked skin of our bodies. But, besides this, most, if not all of these animals are furnished with organs of special touch called tentacles, which serve to collect and convey impressions of the proximity, the form, the hardness, and perhaps other qualities, of those bodies which the animals may desire to investigate. The mantle, also, in many of the Gasteropoda, is fringed with a number of filaments, often curiously branched, which are probably accessary organs of touch.

The sensitiveness manifested by some of the large Gasteropoda, the great Conchs of the West Indies, for example, to the presence of other bodies, even without contact, and which the Rev. Lansdown Guilding attributed to the sense of hearing, may, perhaps, rather be considered as a modification of feeling, capable of appreciating the pulsations of the atmosphere. The experiments of this naturalist, not to be vindicated from the charge of cruelty, are thus described. "I lately suspended," he says, "a number of large Strombi by the spire, that the animal, when dead, might fall from the shell. They had remained in this situation several days, till the body, weak and emaciated, hung down nearly a foot from the aperture, and the eyes had become dim. I found that ​even before my shadow could pass over them, they were aware of my presence, and endeavoured to withdraw into the shell. I then cut off the eyes, with the thick cartilaginous tentacula in which they were lodged, but the animals still continued to be sensible of my near approach, while hanging in this mutilated and painful condition."^[2] Dr. Johnston records a manifestation of feeling, somewhat analogous to this, in one of the common shellfish of our sandy shores. "On a summer evening," he remarks, "I have observed the common Spout-fish (Solen siliqua), extended along the surface of the fine sand in which they burrow, enjoying, apparently, the calmness and mildness of the season, take alarm and instantaneously descend when I was yet distant several yards: and I can explain this and similar facts only on the supposition of the existence of a sense of touch feelingly alive to impressions impalpable to our grosser sense."^[3]

      The respiration is aquatic in most of the Mollusca. The breathing organs, in most cases, resemble in essential points the gills of fishes, consisting of a great number of leaves, often minutely subdivided. They are chiefly formed of blood-vessels, covered with rows of vibrating cilia, by the constant motion of which, currents of water are perpetually hurled along the entire surface of the breathing organ, communicating oxygen, the vital principle, to the blood as they go, through the thin walls of the vessels. In many species, as the Bivalves, the gills form two large comb-like plates; in others they are arranged in the form of a feather; a beautiful tribe, known as naked-gilled,

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