LITMIR.BIZ

      PAPER NAUTILUS.

      Perhaps some of my readers will expect me to include the beautiful Paper Nautilus (Argonauta), among swimming Mollusks; seeing that the poets have claimed for it the honour of teaching navigation to man:—

      "Learn of the little Nautilus to sail,

       Spread the thin oar, and catch the rising gale."—Pope.

      Montgomery, the poet of the ocean, thus beautifully expresses the popular notions concerning it:—

       ​

      "Light as a flake of foam upon the wind,

       Keel upward from the deep, emerged a shell.

       Shaped like the moon ere half her horn is fill'd ;

       Fraught with young life, it righted as it rose,

       And moved at will along the yielding water.

       The native pilot of this little bark

       Put out a tier of oars on either side,

       Spread to the wafting breeze a twofold sail,

       And mounted up, and glided down the billow,

       In happy freedom, pleased to feel the air,

       And wander in the luxury of light."—Pelican Island.

      The accuracy of modern research, however, has proved this to be but a pleasant fable. The Argonaut is a Cuttle-fish, and crawls along the bottom, like its fellows, by means of its slender, flexible, tentacular arms, as represented in the preceding engraving, (fig. 2); while the pair that are furnished with a broad fleshy disk, have an office very different from that of sails, namely, that of forming, repairing, and protecting the thin and papery shell. (See fig. 3.) Its only swimming power appears to be that which it possesses in common with all Cephalopoda, of shooting along in a backward direction, by the force of a jet of water from the funnel, as shown at fig. 1, where it is represented as swimming towards the point a.

Among the Tunicata there are some singular tribes which swim freely in the sea. "The Salpœ, translucent as their native waters, and often united in chains, after a pattern peculiar to each species, are driven along the surface with considerable quickness by alternate contractions and expansions, and by the propulsion they receive from a current of water, which is made continually to traverse the long diameter of the body, sucked in by the posterior aperture, and issuing in a stream through that on the side of the mouth. Hence the body is ​always pushed backwards—a circumstance that has misled some naturalists to describe the posterior aperture for the true mouth. The Pyrosomata are a still more singular family of the same order. Each seeming individual of this genus is, in fact, a numerous colony of little mollusca, every one in its own cell, distinct, yet inseparably connected with its fellows. Collected into the figure of a gelatinous cylinder, open at one extremity and closed at the other, and roughened externally by a multitude of tubercles disposed sometimes in rings and sometimes irregularly, they float in the Australian seas like stars of this lower world, shedding around them a halo of light, brilliant indeed, but surpassed in beauty by those other colours of the creatures which it serves to disclose; colours which come and go at pleasure, glorying as it were, in their subtle changes, passing rapidly from a lively red to aurora, to orange, to green, and to azure blue; a magic scene, compelling more than the admiration of every beholder."^[8]

      Bivalve Mollusca in general have much less power of shifting their locality than Univalves. Many appear to be absolutely stationary, at least during their adult existence. But others, as the Cockle, have a most versatile organ, known as the foot, capable of being protruded from between the valves, which, among its various uses, serves the purpose of locomotion. It is in general applied in this manner. Being stretched out to its utmost extent, its point is made to hook downward into the sand or mud, and the body with the shell is then dragged down by the muscular contraction of the foot. In most cases, this mode of progression ​is sufficiently slow and awkward, but some of the sand-borers are able to conceal themselves thus with surprising rapidity.

      Others of this Class are vigorous leapers; and of some the bounds are so vivacious, forcible and sudden, that they might almost be compared with the flight of a bird, or the shooting of a fish. The Clams or Scallops (Pecten) and their elegant relatives the Limæ, are eminent among our native species for this faculty. The mode in which the leaps are effected is always described as being the opening and sudden closing of the valves or shells; but, in the case of the former genus, I have recently found that the real organ of motion is the mantle. The edges of this being firmly closed, when the interior is filled with water, the fluid is forcibly ejected from the lips, which are relaxed for the purpose at any point according to the will of the Scallop; and by the jet of water striking on the surrounding element, the whole animal is shot to a considerable distance in the opposite direction.

      Most of the MOLLUSCA are, as I have already intimated, inhabitants of the waters, and these are divided between the seas and the freshwaters, in a ratio somewhat like that which these divisions of the waters of our globe bear to each other. Of the marine kinds some dwell permanently and exclusively at the bottom of the deep sea, whence they are to be procured only by dredging. A species of Crania has been brought up from a depth of 255 fathoms. Others inhabit the open ocean, habitually or occasionally swimming on the top of the waves, or resting on the floating sea-weeds of warm latitudes. Many species confine themselves to the vicinity of the shore, where ​each peculiar situation and locality has its proper kinds. The sands, the ooze and mud of harbours, the boulders and loose stones of the wave-washed beach, the sides, ledges, and pools of rocks, uncovered at the recess of every tide, are all inhabited by species peculiar to the respective locality. Some species strictly marine are able to endure protracted exposure to the air, as the Periwinkle and the Limpet, the most familiar of shell-fish, which every visitor to the sea-side habitually sees clustered on the rocks close to the limits of high-water mark.

      The fresh-water Mollusca manifest a similar choice of situation, though a less latitude is permitted for its exercise. Some are peculiar to large rivers, some to estuaries, others to lakes, and yet others to small ponds and ditches.

In general the habits of one species of a genus when ascertained, are found to indicate those of all its fellows of the same genus; as for instance not only is our common Cockle (Cardium edule) a burrower in the ooze at the margin of the sea, but all other species of the genus Cardium have similar habits of life. Yet Mr. Gray has enumerated a considerable catalogue of species, which break this rule, classing them under four divisions. 1st, where species of the same genus are found in more than one kind of situation, as on land, in fresh and in salt-water; 2nd, where one or more species of a genus, most of whose species inhabit fresh-water, are found in salt or brackish water; 3rd, where, on the contrary, one or more species of a genus, whose species generally inhabit the sea, are found in fresh-water; and 4th, where the same species is found both in salt and fresh-water." ^[9]

      ​Of those species which dwell upon the land and breathe the air, most affect moist situations. The common Garden Snail, as is well known, retires to crevices and corners in continued dry weather, where it closes its shell with a temporary door to prevent the evaporation of its vital juices, and patiently waits the return of congenial humidity. The first shower prompts the

Скачать книгу