CHAPTER XV.

MOLLUSCOUS PTEROPODS.

"Natura non facit saltus." LINNÆUS.

The position of the Pteropoda is somewhat unsatisfactory. Their organization in some respects places them below the level of the Gasteropods; but yet the general feeling amongst naturalists has been to assign them a place between the Gasteropods and the most highly organized of the molluscs, the Cephalopods. The number of genera and species is less than that of the other great classes of molluscs.

There are three principal Families of Pteropods. First, the _Cliidæ_, containing Cymodocea, Pelagia, Pneumodermon, and Clio. Second, _Limacinidæ_, containing Macgillivrayia, Cheletropis, Spirialis, and Limacina. Third, _Hyaleidæ_, containing Tiedemannia, Cymbulia, Eurybia, Theca, Cleodora, and Hyalea.

The principal characteristic of the Pteropoda is a membranous expansion situated on the right and left side of their head, from which they take their name of Pteropoda, from ποῦς-πτερὸς, winged feet.

The wings or flappers with which they are provided enable them to pass rapidly through the water, reminding us strongly of the movements of a butterfly, or other winged insect, and like them, their motions are long continued. They advance in this manner in a given direction, while the body or the shell remains in an oblique or vertical position.

These little molluscs may be seen to ascend to the surface very suddenly, turn themselves in a determinate space, or rather swim, without appearing to change their place while sustaining themselves at the same height. If anything alarms them they fold up their flappers, and descend to such a depth in their watery world as will give them the security they seek. They thus pass their lives in the open sea far from any other shelter, except that yielded by the gulf weed and other algæ. In appearance and habits, these small and sometimes microscopic creatures resemble the fry of some other forms of mollusca. They literally swarm both in Tropical and Arctic seas; sometimes so numerous as to colour the ocean for leagues. They are the principal food of whales and sea-birds in high latitudes, rarely approaching the coast. Only one or two species have been accidentally taken on our shores, and those evidently driven thither by currents into which they have been entangled, or by tempests which have stirred the waters with a power beyond theirs. Dr. Leach states that in 1811, during a tour to the Orkneys, he observed on the rocks of the Isle of Staffa several mutilated specimens of _Clio borealis_. Some days after, having borrowed a large shrimp-net, and rowing along the coast of Mull, when the sea, which had previously been extremely stormy, had become calm, he succeeded in catching one alive, which is now in the British Museum.

"In structure," Mr. Huxley tells us, "the Pteropods are most nearly related to the marine univalves, but much inferior to them. Their numerous ganglia are concentrated into a mass below the œsophagus; they have auditory vesicles containing otolithes, and are sensible of light and heat, and probably of odours, although at most they possess very imperfect eyes and tentacles. The true foot is small or obsolete; in _Cleodora lanceolata_ (Fig. 309) it is combined with the fins; but in _Clio_ it is sufficiently distinct, and consists of two elements or _spirals_; the superior portion of the foot supports an operculum. The fins are developed from the sides of the mouth or neck, and are the equivalents of the side-lappets (_Epipoda_) of the sea-snails. The mouth of _Pneumodermon_ is furnished with two supporting miniature suckers; these organs have been compared to the dorsal arms of the cuttle-fishes; but it is doubtful whether their nature is the same. A more certain point of resemblance is the ventral flexure of the alimentary canal, which terminates on the under surface near the right side of the neck. The Pteropods have a muscular gizzard armed with gastric teeth, a liver, a pyloric cæcum, and a contractile renal organ opening into the cavity of the mantle. The heart consists of an auricle and a ventricle, and is essentially opisthobranchiatic, although sometimes affected by the general flexure of the body. The venous system is extremely incomplete. The respiratory organ, which is little more than a ciliated surface, is either situated at the extremity of the body, and unprotected by a mantle, or included in a branchial chamber with an opening in front. The shell when present is symmetrical, glassy, and translucent, consisting of a dorsal and a ventral plate united, with an anterior opening for the head, lateral slits for long filiform processes of the mantle, and terminated behind in one or three points; in other cases it is conical or spirally-coiled, and closed by a spiral operculum. The sexes are united, and the orifices situated on the right side of the neck. According to Vogt, the embryo Pteropod has deciduous vola like the sea-snails, before the proper locomotive organs are developed."

The Pteropods seem to be eminently sociable and gregarious, swarming together in great numbers; they present some analogical resemblances to the Cephalopodæ; but permanently they represent the larval stage of the sea-snails. De Blainville divides the group into two sections, _Thecosomata_ and _Gymnosomata_, the first including the _Hyaleidæ_ and _Limacinidæ_; the second contains one family, the _Cliidæ_. The Hyaleidæ have small horny shells, very thin and transparent, globulous, or elongated, open anteriorly, cloven on the sides, and truncate at the posterior extremity. Their globular body is formed of two parts, the one including the head, bearing two very strong tentacles, and two large fins or flappers in the form of wings, springing from each side of the mouth.

These molluscs are small, and generally of a yellowish-blue or violet colour. They are inhabitants of the deep sea, and rarely seen out of what sailors call "blue water." They plough the waves with great rapidity by the aid of their powerful fins. Certain winds throw them sometimes in great numbers on the shores of the Mediterranean. These little creatures, so inoffensive, and which live together in vast numbers, seem to be an easy and ready-prepared prey, which the great marine animals may swallow by thousands. Twenty species of _Hyalea_ are described as actually living in the Atlantic and Australian seas. Of these _Hyalea gibbosa_ (Figs. 305, 306) and _Hyalea longirostris_ (Figs. 307, 308) are here represented.

The great flappers of _Hyalea tridentata_ are yellow, marked at their base with a fine violet spot. Its shell, plain above, convex beneath, is cloven on the side. The superior part is longer than the inferior, and the transverse line which unites them is furnished with three teeth. This shell is yellow, and nearly translucent. When the animal swims, two expansions of its mantle issue from the lateral clefts in the shell.

_Cleodora lanceolata_ is a delicate and graceful creature; its body, of gelatinous appearance, has a distinct head, with its fins near the neck, notched in the form of a heart (Fig. 309); its posterior part is globulous, transparent, and luminous even in the dark. The animal which inhabits it sometimes shines through the shell like a light placed inside a lantern. This shell is triangular, as in _Cleodora cuspidata_ (Fig. 311), thin, vitreous, and fragile, terminating in a long spine at the base.