CERTAIN marine molluscs, belonging to the Gastropod group, known as sea-slugs, have lost their shells either partially or completely in exactly the same way as the land-slugs. Their bodies are naked like those of the land-slugs, which they resemble in many ways, but they are more beautiful in colour and their backs usually bear a number of warts or small, finger-shaped folds of skin. Some burrow in the sand, others swim in the open waters, but most of them crawl about over the seaweeds, sponges, hydroids and other animals, feeding on them in the same way as land-slugs are found infesting and feeding on the land-plants. As a rule they take on the colour of the seaweed or animal on which they are feeding and for this reason are frequently overlooked.
One of the sea-slugs, owing to the peculiar manner in which
the skin of the back is folded, presents a striking resemblance in miniature to a hare and is known in consequence as the sea-hare. When interfered with the sea-hare pours out from its skin a dense purple fluid, an indelible dye, and this not only obscures the animals movements, in the manner of a smoke-screen, but is also distasteful to its enemies. It has been said, without truth of course, that a human being coming in contact with this fluid would lose all the hair from his head. Sea-slugs, having lost their shells must find some other method of protection, and this is how the sea-hare accomplishes it.
A still more remarkable method of protection is found in the plumed sea-slug. This animal is about three inches long and its back is covered with numerous rows of erect, cylindrical folds of skin. These are partly used for breathing purposes and, in addition, each contains a small organ corresponding to an extent with the liver. The plumed sea-slug often lives on the same ground as certain sea-anemones, resembling them in colour and appearing the more like them by reason of the similarity of the respiratory plumes to the tentacles of the anemones. The anemones being armed with stinging cells are treated with considerable respect by fishes and other carnivorous animals, and the sea-slugs share in this immunity from attack by reason of their resemblance to the anemone. Curiously enough, and as a very poor return for the immunity enjoyed, the slugs feed on the anemones.
Now comes the strange and almost incredible part of the story. Not content with the immunity they already enjoy, the slugs actually arm themselves with the weapons of their victims and use them against foes that may attack them when they are away from the shelter of the anemone’s stings. And this is how it is done. Undigested remains of the anemone’s tentacles pass up into the livers situated in the plumes; the stinging-cells contained in the tentacles remain undigested, pass out of the liver and take up their position in the skin of the sea-slug, whence they can be shot out at enemies when the occasion arises. The truth of this story has often been doubted, but the matter has been investigated again and again and there is no reason now for not accepting it.
In passing, it may be noted that, in addition to this strange method of arming itself, the sea-slug, when pursued, has a habit of throwing off some of its plumes to distract the attention of an enemy while it makes good its escape, new plumes being eventually grown in place of those thrown off.