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THE HABITS OF THE FOUR-HORNED SPIDER-CRAB. 



By H. N. Milligan, F.Z.S. 



Our knowledge of the habits of the Four-horned Spider-Crab 

 (Pisa tetraodon) is exceedingly meagre, partly no doubt because 

 this crustacean is rarely to be seen in aquaria, and the following 

 notes upon the subject may therefore be of interest. These 

 notes are the result of constant observation of about a dozen 

 individuals of this Spider-Crab which have lived from time to 

 time in one or other of the fourteen marine aquaria at the 

 Horniman Museum. Although the facts here recorded relate 

 entirely to these captive individuals, I have tried to add to any 

 usefulness the present paper may possess by referring in their 

 appropriate places to such of the few published records which 

 seem to bear upon those habits I have described. 



Bell has drawn attention to the " extremely slow and 

 measured " movements of the Four-horned Spider-Crabs * ; 

 but their movements are not only slow, they are remarkable 

 for an exceeding cautiousness. Even after several months of 

 captivity these Spider-Crabs will immediately cease to eat or to 

 walk if the front of one of their aquaria (each of which is thirty-one 

 inches long, eighteen inches from back to front, and fifteen inches 

 deep) be abruptly approached. I have seen individuals which 

 have been engaged in tearing food suddenly stop and remain 

 quite still for several minutes with the food (say a piece of sea- 

 weed or a worm) suspended between mouth and outstretched 

 claw, because I have made a sudden movement of my arm in 

 front of the aquarium. They may be seen to behave in this way 

 when on the rocks at the back of the tank, ten or twelve inches 

 away from the glass front, and this suggests that the sight of these 

 Spider-Crabs must be comparatively good. It may be pointed 

 out that the aquaria receive their light from electric lamps above, 

 and that no shadow could fall across the Spider-Crabs from my 

 arm ; therefore, it could not be a shadow which alarmed them. 



It is to be observed that the behaviour of a Four-horned 



* T. Bell, 'A History of the British Stalked-eyed Crustacea,' p. 25. 



