day and examine it through and through for its fauna, and whenever I did this I found many notable cases of mimicry. The color of the younger plants is a yellowish green, while the older stalks are of a more or less dark brown. A luxuriant animal life flourishes on the stems, leaves, and air-vessels of the Sargasso-weed. Little actiniæ, sometimes of a light, sometimes of a dark-brown color, were very numerous on the plants I most closely examined; often so thick as to completely cover the stems. On the same plants, I also found numerous specimens of small, naked snails. These minute gasteropods, a centimetre or a centimetre and a half long, bore on their backs numerous retractile tentacles, arranged in cross-rows at various intervals. In color, they were of various shades of brown, like the actiniæ; when they drew themselves up so that the tentacles stood thickly together, they so much resembled the actiniæ that it would be a matter of difficulty to a person not acquainted with both animals to tell them apart. Another snail, whose tentacles were arranged in rows along each side of the back, was still more difficult to distinguish when any danger threatened it from the actiniæ. Of what use can the resemblance to the actiniæ be to the little mollusks? They are, it is true, great eaters of the actiniæ, for I have seen one of them devour four or five of those animals in an hour; but it does not appear that their access to them is greatly facilitated by the resemblance, for the actiniæ are so confined by the limitation of their movements as to be unable, in any case, to escape their more facile enemies. We are, therefore, reduced to consider the likeness a case of mimicry for the protection of the snails against animals which pursue them, but avoid the actiniæ, whose nettle-cells are by no means pleasant morsels. But as I have not been able to discover what special enemies the snails have, and whether they really dislike the actiniæ, my attempted explanation must remain an attempt, to be confirmed or disproved by some future observer in the Sargasso Sea, who can begin where I have had to leave off. Other cases of mimicry on the part of mollusks have come under observation. According to Dr. H. von Ihering, the Chromodoris gracilis lives associated with a sponge (Suberites), and is colored like it, blue.
On the same sea-weed I found other larger mollusks, which, not resembling other animals, so strikingly resemble the forms of the stems and leaves of the plants that it is extremely difficult to find them in the tangle of brush. They have developed flaps all around their bodies, before and behind, and on either side, the edges of which are irregularly serrated, with the tips of the serratures of a brown tint like the older alga-stems. The surface of the flaps, and of a part of the rest of the body, is beset with numerous small similarly brown tipped teeth, while the color of the animal as a whole is olive-green, like that of the plant in which it lives.
Moritz Wagner regards the phenomena of mimicry as the consequence of an innate caution in the animal, that causes it to choose those
