Figs. 305 and 306. Hyalea gibbosa (Rang.). Figs. 307 and 308. Hyalea longirostris (Lesueur).
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.
Fig. 309. Cleodora lanceolata (Lesueur). Fig. 310. Cleodora compressa (Eydoux and Souleyet).
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.
Fig. 311. Cleodora cuspidata (Bucc.)