green color, which well adapt them to abiding among the seaweeds. In the intestines of many specimens which I have examined for that purpose I have found among fine grains of sand and mold dubious remains of sea-plants and little shells of swimming mollusk-larvæ.
Many sea-butterflies are naked, having their spindle-like bodies, instead of shells, covered only by a sack-like skin. The laterally fixed wings are sometimes drawn back into pockets, and over them rises a roundish, somewhat depressed head-part, which is occasionally provided with appendages bearing hooks or suckers. To them belongs the above-mentioned violet-colored Pneumodermon of the Mediterranean Sea, which, when danger is impending, envelops itself in a white cloud of slime that is secreted in numerous glands, but is soon exhausted.
A species occurs in the northern seas which, together with a little butterfly, Limacina arctica (Fig. 8)—a species having a somewhat spiral, transparent shell—comes into remarkable direct relations with man. The little Limacinas appear in immense swarms in the polar seas, and the not less numerous naked Cliones, Clione borealis (Fig. 7), which are much larger and inflict grievous destruction upon them. In the Mediterranean Sea the Cliones are represented by the related genus, Clinopsis Krohnii (Fig. 6). The polar voyager. Captain Halböll once tried to bring some living Cliones to Prof. Eschricht, in Copenhagen, for examination. Knowing that they were carnivorous, he fed them with reindeer meat, which they ate greedily at first; but, although he changed the water frequently, he was not able to keep them alive more than eight days, and had to bring them preserved in alcohol. But Eschricht made a very satisfactory research upon them.
The Limacinas eat little crustaceans, the Cliones eat the Limacinas, and both are consumed by the ton by whales. The Greenland whale appears to live almost exclusively on the two species of sea-butterfly, which it has to swallow in immense quantities to fill its capacious maw. It eats also other pelagic small fry and crustaceans as side-dishes.
These are only indirect relations in which the sea-butterflies inhabiting all seas stand to man. But they are important enough. Without whale-food, no whales; without these, no blubber to grease sailors' water-proof boots and overalls; and without boots and southwesters, no sailors and high-sea fishermen; and without whales, no whalebone, no parasols and umbrellas and corsets, which were not worn by the beauties of ancient times, because they were limited to the productions of the Mediterranean Sea, where there are no Greenland whales. But chains of this kind can be found everywhere.
The older French naturalists—D'Orbigny, Péron, Lesueur—