(1478). Nearly allied to the Gasteropods in their internal organization, but differing from them remarkably in the character and position of their locomotive apparatus, are the Pteropoda - a class of mollusks of small dimensions, but met with in astonishing quantities, at certain seasons, in various parts of the ocean. So numberless, indeed, are these little beings in those regions where they are common, that the surface of the sea seems literally alive with their gambollings; and thus the store of provisions necessary to render the waters of the ocean habitable to animals of higher grade in the scale of life is still further increased. The great character that distinguishes the members of the class upon the investigation of which we are now entering is derived from the structure of their organs of locomotion. These are only adapted for swimming, and consist of two broad and fleshy expansions, attached like a pair of wings to the sides of the neck, and forming moveable fins - enabling the little beings to dance merrily among the foamy waves, now sinking, and again rising to the surface, until some passing whale, opening its enormous jaws, engulfs multitudes of such tiny victims, and hence derives the materials for its subsistence.

A wing.

A wing.

The foot.

The foot.

(1479). Several distinct genera of Pteropoda have been established by zoologists, and some important modifications have been detected in their organization, although in all of them the lateral alae form the instruments of progression.

(1480). The Clio borealis, anatomized by Cuvier*, and more recently and completely investigated by Professor Eschricht of Copenhagen 1, is one of the species best known, as well as most abundantly met with; it is therefore by a description of this Pteropod that we shall proceed to introduce the reader to the general facts connected with the history of the animals under consideration.

Clio borealis; represented at A in a state of repose.

Fig. 277. Clio borealis; represented at A in a state of repose, while B, C exhibit the various external appendages fully protruded: a a, wing-like oars; b, hood retracted; g, bladder-like organ; h, penis; k, tentacle; o, globular protuberances; s s, conical appendages.

(1481). The body of the Clio is about an inch in length, of an oblong shape, and terminating posteriorly in a point; while at the opposite extremity there is a little head supported upon a short neck, and furnished with delicate retractile tentacles, apparently instruments of touch. The locomotive organs, as the name of the class imports, consist of two delicate wing-like appendages (fig. 277, a a) attached to the two sides of the neck, by means of which, as by a pair of broad fins, the Pteropod rows itself about with facility. But the two aliform membranes, although externally they appear separate instruments, are, as we are assured by the observations of Professor Eschricht, but one organ, being made up entirely of muscular fasciculi, which pass right through the neck and spread out on each side in the substance of the wing, forming an apparatus exactly comparable to the double-barrelled oar with which the Greenlander so dexterously steers his kajac, or canoe, through the very seas inhabited by the little Clio we are describing.

* Memoire sur le Clio borealis.

1 Anatomische Untersuchimgen uber die Clione borealis, von D. F. Eschricht. Kopenhagen, 1838, 4to.

(1482). The head of one of these animals is surmounted by various organs appropriated to different offices, and some of them not a little remarkable from the amazing complication of structure which they exhibit. On each side of the oral opening are three conical appendages (fig. 277, c, s), that to a superficial examiner might appear to be mere fleshy tentacula; but in reality they are instruments of prehension, of unparalleled beauty and astonishing construction. Each of these six appendages, when examined attentively, is seen to be of a reddish tint; and this colour, under the microscope, is found to be dependent upon the presence of numerous minute isolated red points distributed over its surface. When still further magnified, these detached points are evidently distinct organs, placed with great regularity, so as to give a speckled appearance to the whole of the conical appendage; and their number, at a rough guess, may be estimated at about three thousand. Every one of these minute specks is, in fact, when more closely examined, a transparent cylinder, resembling the cell of a polyp, and containing within its cavity about twenty pedunculated disks, which may be protruded from the orifice of their sheath (fig. 278, c), and form so many prehensile suckers adapted to seize and hold minute prey. Thus, therefore, there will be 3000 x 20 x 6=360,000 of these microscopic suckers upon the head of one Clio - an apparatus for prehension perhaps unparalleled in the creation.

(1483). When not in use, the appendages referred to are withdrawn, and concealed by two hood-like fleshy expansions, which, meeting each other in the mesial line, completely cover and protect the whole of this delicate mechanism, as represented in (fig. 277, a).

(1484). Still, however, even when the hoods are drawn over the parts they are intended to defend, the Clio is not left without tactile organs wherewith to examine external objects; for each valve of the hood is perforated near its centre: and through the apertures so formed, two slender filiform tentacula (fig. 277, c, k), somewhat resembling the feelers of a Snail, are protruded at the will of the animal; and by means of these it is informed of the presence of food, and instructed when to uncover the elaborately-organized suctorial apparatus destined to seize it and convey it into the mouth.