264 L. A. BORRADAILE. 



are restricted, and within which they are everywhere abundant on coral reefs. They find shelter 

 on various sessile animals, especially branched corals, from which they are usually not to be 

 dislodged except by breaking up the stock, and it is very remarkable that they are found only 

 on living corals, and in these only on those branches which are still alive. Yet they do not 

 eat or in any way damage their host, contenting themselves with deriving protection from its 

 stinging-cells'. In correspondence with this habitat, their legs are rather slender, and have 

 end-joints which can be moved through a considerable arc, so as to form with the next joint 

 a kind of hook, serviceable for grappling the branches of the host. These end-joints (Figs. 42 B, 

 58 c) are remarkable and interesting structures when the habitat of the crabs is borne in 

 mind. Besides the peculiar flange working on the joint before, which has been described 

 above on p. 242, and is probably an adaptation to clambering, there are several other modi- 

 fications, which seem intended to give the animals a better foothold on the rugged and yet 

 slimy surface of the coral. Thus the joint is covered with hairs, and among these are a 

 number of stout, moveable prickles. The claw at the end of the joint is bent sharply forming 

 a hook, and is smooth and pointed in Quadrella, while in Trapezia it is broad and shovel- 

 like, and bears above two rows of structures resembling the prickles but rounded at the end. 

 It is likely that the smoothness of the bodies of these crabs is due to the same cause as 

 that of other semiparasitic Decapods, such as the mussel-prawns and crabs (Pontonia and 

 Pinnotheres), and that this cause is the need of inconveniencing as little as possible the 

 animals which give them shelter. Trapezia and Tetralia especially affect corals, but Quadrella 

 has been taken on Alcyonarians of various kinds, and even among pearl-mussels". 



The Trapeziinae are exceptional among Xanthids on account of their tendency to form 

 varieties, and present the feature, unusual in crabs, of definite colour forms. It is possible that 

 some of these forms may be physiological phenomena of the same kind as the colour-phases 

 of prawns^, but others are associated with structural features, and cannot well be of this nature. 

 It is also important to note that, in Trapezia, they bear no relation to the colour of the coral. 

 For the present it were well that none of them were overlooked, and I have therefore revived, 

 in two cases, colour names which had been dropped, without, however, intending any implica- 

 tion as to the nature of the phenomena. 



The synonymy of this group is extremely intricate, and the authority followed here is 

 Ortmann, in his late revision [^Zool. Jahrh. Si/st. x. ii. p. 201 (1897)]. 



Genus Trapezia Latr., 1825. 

 81. Trapezia ferruginea Latr., 1825 (Fig. 41 F, 42 B). 



i. Var. typica Ortm., 1897, Alcock, ill. p. 220. 



On reefs and in lagoons down to 35 fathoms in Goifurfehendu, Male, Fadifolu, Addu, 

 Minikoi, South Nilandu, Suvadiva, and South Male Atolls. 



ii. Var. dentata (Mackay), 1838. Ortmann, loc. cit. p. 204. 



On reefs at Male, Goifurfehendu and Fadifolu Atolls and in Suvadiva lagoon in 25 fathoms. 



' I am much obliged both to Mr Stanley Gardiner and to among the living branches, nor was there any evidence of 



Mr Forster Cooper for information confirnnng tliese .state- damage to the coral caused by them. 



ments. From first to last a very large number of coral - Smith, Proc. ISoston Soc. Nat. Hist. xii. p. 288 (1869). 



stocks were broken up by the members of the expedition, ■' Gamble and Keeble, Q. J. M. S. XLtii. Pt. 4. 



and in none of these were Trapezius found anvwhere but 



