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. [Illustration: 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. [Illustration: FIG. 114.--1, _Dentalium entalis_, natural size; 2, shell magnified, and broken to show animal within; 3, animal projecting from the shell; 4, animal from below, magnified; 6, same from above; 5, same, showing internal structure.] XIII. THE CUTTLEFISHES