has left the door open behind it."[1] It may be contended that this shows only mimicry in the habitation, and not in the appearance of the animal itself; or, again, that "aggressive" rather than "active" should be the qualitative term applied to this mimicry; but we can refer to instances where animals disguise their own bodies in a similar manner, and with a like intelligence, to these Spiders. The little Æsop's Prawns (Hippolyte (virbius) varians, Leach, and H. fascigera, Gosse) may perhaps be cited as practisers of active mimicry. Prof. Herdman, in 1893, described four variations of H. varians, each agreeing in hue with the colour of its special habitat,[2] and was inclined to accept the fourth possibility of explanation which he suggested, viz. "The young may be very variable in tint, and then, by the action of natural selection, such as do not agree in hue with the surroundings will be eliminated." Mr. James Hornell, at the Jersey Biological Station, has made a further series of experiments with these species, and has accepted the third postulate of Prof. Herdman, viz. the "adaptability may be retained throughout the rest of their lives, and the adults may change hue upon change of environment." Mr. Hornell found that a pale olive-brown H. varians taken from amid similarly coloured seaweed became of a vivid green within an hour when placed with Enteromorpha, and the same specimen changed to a pinkish red within three hours when placed amid Delesseria. Again, red-coloured specimens of the same species from amongst tufts of red weeds changed to green during a single night when placed with Enteromorpha, or with Cladophora, and back again to red within four hours when placed once more amid red weed. This change of hue took place as rapidly in the dark as in the light. The weeds affected by the smooth-skinned H. varians, in the great majority of cases, are smooth in surface, and not overgrown with foreign matter. "In marked contrast, the body of H. fascigera is ornamented with tufts of brush-like hairs, and if a spray of the coarse Corallina, where this species makes its home, is examined, the stems are found covered with a multitude of abodes of tiny "messmates," porcelain-like coils of the little tube-worm Spirobis, dull-looking cylinders tenanted by that lovely miniature Sabeliid,
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