Fig. 111.—A sea slug (Dendronotus).
In floating on the borders of the Sargasso Sea, the vast sea of weeds in the South Atlantic, I found numbers of a beautiful sea slug (Fig. 111) which so resembled the weeds in shape and color, a rich olive green, that it was almost impossible to distinguish it, except when very close to the surface. They have attractive names, as Doris, Tritonia, Æolis, and Aplysia, and are among the wonders of the great belts of kelp which surround the continent. I once found a slug at Santa Catalina which was a vivid, almost iridescent purple; another was yellow; but the most interesting was Aplysia, a giant two feet long, which I kept in an aquarium. It weighed nearly eight pounds, could lengthen itself out to a distance of nearly three feet, or contract into a dark, olive-hued ball, scarcely six inches across. It took sea lettuce from my hand, eating with avidity, and when disturbed emitted a purple ink which filled the water and hid the monster "sea hare" from view. It laid its eggs on the sides of the tank in long chains, but if not well fed, exhibited a decided cannibalistic tendency, devouring its own progeny. This animal had an enormous foot by which it crept rapidly along, and it invariably protected itself by imitating the color of the bottom upon which it rested.
Fig. 112.—Onchidium, a sea slug with eyes on its back.
One of the most interesting slugs is the Onchidium (Fig. 112). According to the naturalist Semper, it has upon its back numerous eyes, which enable it to see from above. It is a mud-loving form, common in our new provinces, the Philippines, where the heat is intense and the water warm. One of the land slugs, Limax noctiluca, emits light; and the eggs of another, Arion, have been noticed to be luminous for nearly two weeks after being deposited.
Fig. 113.—A, B, C, pteropods; D, young of Cleodora, all highly magnified.
Nearly all these shells are slow-moving animals, but there are others, the pteropods (Fig. 113), which are swimmers. The veritable fairy craft of the sea, they are housed in shells of dainty structure and moving by singular winglike fins, which give them the name of ocean butterflies. They have the property of phosphorescence to a remarkable degree. Cleodora (Fig. 113, D) emits a soft light which gleams through the delicate shell like a light in a lantern. In swimming it moves its fins up and down very much like a butterfly, so that they touch at the top. As delicate and dainty as this little creature is, it has a marvelous arrangement for seizing prey, each tentacle having about three thousand transparent cylinders, each of which contains twenty stalked suckers. As there are six tentacles on each sucker, Cleodora can grasp its prey with three hundred thousand hands. Equally dainty in its way is the sea snail, Ianthina, a violet shell of great delicacy, whose foot develops a raft which resembles a mass of soap bubbles, so the violet snail floats upon the surface of tropical and semi-tropical seas. I have seen the shores of the keys of the Florida Reef lined with an undulating ribbon of these shells after a storm. When touched they emit a rich violet ink which lasts a long time as a stain. A small species of Ianthina is found in the winter on the southern Californian shores, and beneath the attractive float will be found the eggs.
