1 905,] BEARING ACTINIANS IN THEIR CLAWS. 003 



oral disc and tentacles turned upAvards, this being the usual rela- 

 tionship when first captured. 



Whenever an actinian was removed from the claw of a crab, 

 certain cleansing-opei-ations on the part of the latter invariably 

 took place. Ordinarily the claws have particles of debris adhei-ing 

 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 move- 

 ments over them. So effective were these, that within a shoi-t 

 time the claws were altogether cleansed of any adhering foreign 

 particles and presented a much fresher appearance. 



Reactions. 



Under ordinary circumstances the crab when at rest holds its 

 two chelipeds bent towards itself, and the actinians are inclined 

 upwards and outwards with the tentacles fully expanded, thus 

 masking to a certain extent the aiiterior part of the crab. When 

 walking the chelipeds are held more forwards, and the actinians 

 are then presented in what can be best described as a threatening 

 attitude. It would be impossible for any animal of moderate 

 size to molest the crab in front without touching the polyps. Some- 

 times the claws are held downwards so that the polyps touch the 

 bottom of the vessel, and on being dragged over it the muciis with 

 which the body is covered leads to the adherence of debris. 



If any part of the crab be touched in front, the reflexes are such 

 that the chelipeds are at once extended in the direction whence the 

 stimulus proceeds, the polyps being thereby raised and presented 

 in a defensive attitude. Likewise when irritated from the side, 

 both chelpe are turned laterally, the polyps again being directed 

 towards the region whence the stimulus comes. Similarly, if 

 touched on the upper surface of the carapace or posterior part of 

 the body, the actinians are turned upwards and backwards : in 

 fact, by varying the part stimulated, the chelipeds can be made 

 to turn through about two-thii'ds 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 

 such a nature that the claws bearing the actinians are turned 

 towards the part irritated, thereby placing the polyps in a position 

 most favourable for defence or offence. The reflexes are usually 

 rapid and continue for some time, first in one direction and then 

 in another, according 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 out, always occurs 

 as a preliminary measure : retreat on molestation is by no means 

 so readily resoi"ted to as in most crabs. 



It is of much importance to find that exactly the same responses 

 Prog, Zool. See— 1905, Vol. II, ^o. XXXIY, 34 



