MARINE CRUSTACEANS. 239 



grows. The massive, compact form of body and the strong cuticle are, no doubt, adaptations 

 to a life spent in positions where they are surrounded with hard, stony objects, and exposed 

 at times to surf and strong currents, and at others to enemies which they are unable to 

 escape by swimming like the Portunidae. 



From this typical body-form there are, of course, deviations in various directions, of 

 which a few only can be mentioned here. Such genera as Chlorodius (Fig. 41 1)) and Pseu- 

 dozius (Fig. 45) are flatter and lighter built, with longer legs, though the cuticle is still strong, 

 and they are more often to be found in situations like the interstices of coral stocks, where 

 they must maintain their position by the use of their limbs, rather than by their weight 

 or by wedging themselves fast, like the heavier genera. Yet even these' are far from being 

 true coral crabs, restricted to the living jjarts of the stock, like Melia (Fig. 49), Bomecia 

 (Fig. 41 E), or the Trapeziinae (Figs. 41 F, 58), on which some remarks will be found below-. 

 Another very characteristic habitus is that of Pilumnus (Figs. 46 — 48), slightly built, very 

 hairy, and often with squarish, rather than oval, bodies, falling very steeply in front. 



As to the meaning of the numerous, and often very beautiful, features by which the 

 Xanthids are distinguished in detail, it is as yet impossible, in most cases, to say anything. 

 Why, for instance, are the species of Actaea bejewelled with the most beautiful pearly granules 

 (Figs. 53, 54) ? And why do these granules become sharp thorns in some species (Fig. 56), 

 and flatten themselves into leaflets in others (Fig. 55) i Why should the back of Euxanthus 

 (Fig. 41 C) be rough and uneven, or that of Carpilius (Fig. 41 B) glassy smooth ? Why is 

 Liomera barrel-shaped (Fig. 52) and Pseudozius often as flat as a board (Figs. 43, 44)? 

 Before these questions can be answered, much more must be known about the habits of 

 the crabs. We may, however, indicate some directions in which it will be well to look 

 for the answer. The texture of the back and legs, and their nakedness or hairiness, will 

 perhaps be found to depend on the necessity of friction with the surrounding objects when 

 the animal wedges itself firmly into its hiding-place, or of presenting a smooth surface to 

 waves and currents, so that they may pass over the animal without sweeping it away; or 

 again to the need of disguise, either by a covering of fine silt, held by hairs or bristles 

 (often a very effectual device) or by adherent organisms which will not grow on a smooth 

 surface, or even [Actaea cavipes and fossulata (Fig. 41.4)] by a likeness to water- worn pebbles 

 of coral. The shape of the hands may be adapted to the food, or to defence against some 

 special enemy, or — in correspondence with that of the front — to breathing among foreign 

 particles of various grades of coarseness', or in some cases seems to be a sexual character. 

 The remarkable hoof-like ends to the fingers (Fig. 57 C), which turn up again and again in 

 different genera', may serve to give their owner a foothold among the rough coral branches, 

 or may gather food in some special way, like the tufted, spoon-like fingers of the pra\vn 

 Atya. The shape of the front depends on the way in which the antennae are used, and on 

 the need of keeping open a way for the breathing stream, a function which it shares with 

 the chelipeds''. And lastly the shape of the end-joints of the walking legs, which shows a 



' Except, probably, P. coralliophilus and P. triunguicu- than it has yet received. Another instance of it, mentioned 



latiis (p. 241 S.). below (p. 242), is the pulley arrangement at the end of the 



- pp. 249, 263. walking legs. Of course the closure of the orbital gap has 



' Garstang, Q. J. M. S. XL. p. 211. happened over and over again in various groups of Crabs, and 



' This phenomenon of the occurrence of the same feature the loss of the mandibular palp and the epipodites among 



in groups which cannot be related to one another is not Prawns is a similar case. 



uncommon among Decapods, and is worthy of more attention 



G. 31 



