to recovery or removal. In its fall, accidental or purposed, an air-bubble is emitted, probably from the branchial cavity, which rises slowly through the water, and as the snail has enveloped it with his slime, this is drawn out into threads as the bubble ascends; and now, having a buoy and bladder whereon to climb to the surface, it waits suspended until that bubble comes into contact with the weeds that everywhere float around."[6]
A species of Cerithium, found at the mouths of rivers in tropical countries, has the silkworm-like habit of spinning threads, by which it suspends itself from the mangrove-roots; and our own, freshwater Snails have the power of suspending themselves in the same manner. Mr. Warington, in an interesting paper on the habits of some aquatic animals kept in confinement, thus records the curious fact:—
The wide expanse of ocean from the equator to the poles is tenanted by a class of swimmers, small, indeed, in the number of its species, but countless in the hosts of individuals of which they are composed; the Pteropoda. Some of these inhabit shells, which for delicacy and transparency, exeel the thinnest glass. They possess a pair of large membranous fins, which closely resemble the wings of a butterfly, and by using these organs in a flapping manner, the little animals swim briskly about.
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
