242 



L. A. BORRADAILE. 



natural to suppose at first that this was the female of the crab whose male we have just 

 described. Further examination, however, showed differences between the two so great that 

 they must belong to different species. The second specimen is described below under the name 

 of P. triunguiculatus. 



I am much obliged to Mr Edwin Wilson, F.E.S., for calling my attention to a very in- 

 teresting structure which he discovered on the legs of this crab (Fig. 43 b — d). As usual, the 

 last joint (dactylopodite, end-joint) is held in a deep notch at the end of the one before it 

 (propodite). But in the present case it bears on the hinder side a flange, and on the flange 

 a knob, which works in a groove on the propodite. At the inner end of this groove is a 

 small pit, into which the knob slips when the joint is fully flexed, so that the end-joint is 

 held firm in this position, thus providing the crab with a hook, by which, no doubt, it keeps 

 its position on the coral. P. triunguiculatus has a less perfect form of the same apparatus 

 (Fig. 44 c). In it, the flange on the last joint works against a special process of the one before 



Fig. 44. Pseudozius truingtiiculatits, symbiotic with a coral; a. whole animal, b. outside of greater hand, 

 c. hinder side of end of walking leg, d. fore side of the same. 



it, but this process has only a smooth surface and no gi-oove. A small spine under the pro- 

 podite seems placed in this position to prevent the end-joint from being unduly flexed, and 

 no doubt the other spines give a foothold to the animal. On the whole, these structures, as 

 well as the general appearance of the body and limbs, give the impression that the crabs are 

 indeed symbiotic with the coral, living always on its surface, but take refuge in the Crypto- 

 chirus holes at times only. 



Interestingly enough, the same structure, in its simpler form of a flange on the end-joint 

 working along a smooth path on the joint before it, turns up again in the coral crabs of the 

 Trapeziinae and in Domecia, though not in Melia, which crooks the whole leg and not the 

 last joints only, and is also present in Chlorodopsis (Fig. 57 c) and Phymodius, which, without 



