| OCR Text |
Show 1905.] BEARING ACTINIANS IN THEIR CLAWS. 5 0 9 reach, passing them towards the middle of the disc, from which, however, they are abstracted by the ambulatory limbs of the crab. Enemies to the crab, too large to be held by the tentacles of the polyps, may nevertheless be warned off by the stinging-cells of the anemone emitted on irritation. A careful consideration of all the circumstances justifies the view that the crab will secure much of its food through the activity of the anemones, and, further, that the latter will exercise a protective influence upon the crab against larger enemies. The advantages to the actinian appear largely negative. As Mobius suggests, the movements of the crab will serve to bring the actinian into the neighbourhood of more prey, but its chances of ultimately appropriating to itself much of this seem very small. The feeding experiments demonstrated very clearly that it is only rarely that the actinians succeed in ingesting their food ere it is withdrawn by the crab. In the case of the actinians Sagartia and A damsia, commensal with hermit crabs, it is usually considered that the polyps secure fragments of the food torn up by the masticatory appendages and slipping away, but it is not likely that this occurs with Melia. Independently of the actinians, the crab can only obtain such food as may be lying upon the sea-floor and incidentally come upon the maxillipeds and the ambulatory limbs. The acquisition of such a peculiar commensal habit on the part of two wholly distinct types of crabs, Melia and Polydectus, correlated, in the case of the former at least, with a diminutive size and partial loss of activity on the par-t of the chelipeds, does not admit of ready explanation. Among the activities of other crustacea there appear to be no examples which help us to understand how such behaviour and structural peculiarities have become established-no simpler or intermediate stages which suggest the lines along which the evolution has taken place. In the well-known instances of masking-crabs (Stenorhynchus, Dromia) we have the tearing away of suitable objects, such as zoophytes, algse, and sponges, which are then affixed to the shell; but the instinctive processes involved therein are less complex than in the cases under consideration. In the latter the ordinary aggressive and tearing functions of the chelipeds are replaced by those of merely holding a living example of another group of organisms. Even the seizure by a crab of an anemone and the affixation of it upon a gastropod shell, as in the well-known hermit crabs Pagurus, and the actinians Sagartia parasitica and Adamsia palliata, involves much less of a departure from the usual activities of crustacea. As in so many morphological and physiological phenomena in nature where intermediate stages are not forthcoming, it is difficult to see how such an instinct could have been acquired or evolved by slow degrees. For instance, while holding the actinians the crab could not at the same time employ its claws for the usual purpose of seizing and conveying food to its mouth. One is constrained to think of mutation as a possible explanation |
