THE SEA BUTTERFLIES - CLASS PTEROPODA. A' PELAGIC' group of mollusks, 'reaching shore only by accident, as when storm-driven. They live in communities, in all seas; the Atctic species are the most highly coloured. They rise to -tile surface at' twilight; rarely:specimens come up daytime. They feed 'Upon microscopic mollusks and crustaceans.' • The pteropbds are all small mollusks, naked or with small, transparent shells, internal or external.. Some are trumpet shaped, some cylindrical, with needle-like shells. Others are or 'globular. 'Shells like those of the pearly nautilus, the purples and the Hungarian cap occur :among the spiral forms. The foot is dilated into two 'wing-like' swimming disks, or these disks occur as accessory organs of locomotion, the focit being rudimentary. 'The/position of the body. in swimming is "wrong side up," the abdomen uppermost. The head has tentacles which bear organs of hearing and smell, but not of. sight. The large proboscis has a lingual ribbon armed with recurved spines. There are sometimes grasping organs. Gills are internal or ex ternal. Young pteropods swim by a velum until the adult swimming lobes appear. In all the young have shells.
The interesting genus, Firola, has a few species, with slim fusiform bodies, propelled by a ventral and a caudal fin. The gill rises unprotected above the tail. These creatures exhibit their entire structure without reserve. The circulation of the blood is traceable through the transparent tissues. Well devel oped hearing organs enable the to detect enemies. Their flight is swift, and the loss of the head does not seem to deter them at all.- Adults have no shells.
They inhabit the Mediterranean Sea, the Atlantic and Pacific oceans.
Spirialis Flemingii, seen off Nahant' in considerable abun dance in 1863, were studied Alexander Agassiz. He observed that they came to the surface at high tide when it occured directly 241 The Sea Butterflies after dusk, but were rarely seen in daytime, or after ten o'clock at night. In the aquarium they crept along the bottom by
means of their wing-like appendages. In daytime they would rise a few inches, then fold their wings and drop. But by night they came up and gambolled at and near the surface, flapping rapidly about like butterflies.
Whales swallow enormous numbers of pteropods of the genera Limacina and Clio, which swarm at twlight, in Arctic seas, colouring the surface for miles.
Genus CARINARIA, Lam.
Shell cap-shaped, thin, glassy, brittle, covering the stalked nucleus; body large, oblong, gelatinous, with two fins, a well developed head, eyes, tentacles and a strong snout with toothed tongue. The gills, which protrude from under the shell, are feathered.
The shells of these strange ocean swimmers were known long before the animals had been observed. One of them looks like the cap of some fairy harlequin; the peak surely ought to dangle a tassel of spun glass. Unnecessary seems the shell indeed, as the creature darts about, seizing small pelagic animals with its great proboscis.
C. Atlantica, Ads. and Rve., inhabits the North Atlantic. The shell is depressed and the apex decidedly coiled. Similar to it is the Mediterranean species, C. fragilis, Bory.
C. vitrea, Lam., larger, and with shell attenuated to a high, sharp peak, is found in the Indian Ocean.
Genus ATLANTA, Les.
Shell nautiloid, minute, glassy, compressed, keeled; aperture small, notched; operculum lamellar; animal able to withdraw into shell; gills contained in a dorsal mantle cavity; head large; eyes conspicuous; ventral fin fan-like, provided with a fringed sucker.
A. turriculata, d'Orb., is a lively little mollusk. It swims, shell downward, with sudden jerks, by means of the fin-like foot, and the tail fin. It rests when tired by attaching the disk to floating objects. "Glassy Nautilus" it was called before the structure of the soft parts was known.
Habitat.— Warm parts of the Atlantic Ocean.
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