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Show 1 9 0 5 .] BEARING ACTINIANS IN THEIR CLAWS. 5 0 3 oral disc and tentacles turned upwards, this being the usual relationship when first captured. Whenever an actinian was removed from the claw of a crab, certain cleansixig-operations on the part of the latter invariably took place. Ordinarily the claws have particles of debris adhering to the hairs with which they are provided, as well as to the general surface of the skeleton, the amount being undoubtedly increased by the presence of mucus from the actinian. On the chelipeds becoming unoccupied they were turned towards the mouth, and the masticatory appendages, which are richly provided with bristles and spines, at once began a series of scraping or combing movements over them. So effective were these, that within a short time the claws were altogether cleansed of any adhering foreign particles and presented a much fresher appearance. R e a c t io n s . Under ordinary circumstances the crab when at rest holds its two chelipeds bent towards itself, and the actinians are incliixed upwards and outwards with the tentacles fully expanded, thus masking to a certain extexxt the anterior part of the crab. When walking the chelipeds are held more fox'wards, and the actinians are then presented in what can be best described as a threatening attitude. It would be impossible for any animal of moclei'ate size to molest the crab in front without touching the polyps. Sometimes the claws are held downwards so that the polyps touch the bottom of the vessel, and on being dragged over it the mucus with which the body is covered leads to the adherence of debris. If any part of the crab be touched in fi'oxit, the i'eflexes ax e such that the chelipeds ax*e at once extended in the direction whence the stimulus proceeds, the polyps beixig thereby raised and px'esented in a defexxsive attitude. Likewise when irritated from the side, both chelae are turned laterally, the polyps again being directed towards the x'egion whence the stimulus comes. Similarly, if touched on the upper surface of the carapace or posterior pai't of the body, the actinians are turned xxpwards and backwards : in fact, by varying the part stimulated, the chelipeds can be xnade to turn thx'ough about two-thirds of a circle in a vertical direction, and in addition they can perform complex lateral movements. In every case, it may be said that the responses of the crab are of sxxch a nature that the claws bearing the actiniaxis are turned towards the part irritated, thereby placing the polyps ixi a positioxi most favourable for defexice or offexice. The x'eflexes are usually rapid and continue for some time, first in one dix'ection axicl thexi in another, accordixig to the region irritated. At the same time the crab generally moves away from the stimulus, backwards or forwards, though the claw reaction, the striking oixt, always occurs as a prelixninary measure : retreat on molestation is by no means so readily resorted to as in most crabs. It is of much importance to find that exactly the saxxie responses P roc. Z ool. Soc.- 1905, V ol. II. No. XXXIV. 34 |