LAND CRUSTACEANS. 



73 



The family Coenobitidae, containing besides Coenobita the Robber-Crab Birgus, differs 

 from the Pagurids, or hermit-crabs of the sea', in the following external features : — 1. The 



Fig. 18. Last two abdominal segments of Coenobita clijpeatiis. A. Dorsal view ; 

 B. Ventral view. 1. Valve guarding the anus. 



Structure of the antennules (see above). 2. The structure of the antennae (see above). 3. The 

 greater compression of the fore-part of the body. 4. The habitat — on land. 



The genus Coenobita differs from Birgus in the form of the abdomen, which in Birgus 

 is short, untwisted, provided with broad terga, and not carried in a shell. The points in 

 which the outward features of Coenobita vary from species to species are small and un- 

 important. The most interesting is perhaps that which separates C. clypeatus Latr. from 

 the rest of the genus. In this species the vestige of the antennal scale is still loosely 

 articulated with the stalk. In all the others it is fused. C. diogenes (Catesby) is distin- 

 guished by its cylindrical eyestalks from the other species, in which these structures are 

 always compressed. Of the remaining four forms, C. rugosus H. M.-Edw. and C. perlatus 

 H. M.-Edw. are distinguished from C. compressus Guerin and C. spinosns H. M.-Edw. by the 

 elongation of the basal joint of the fifth leg of the male into a genital process, and by 

 the presence of stridulating ridges on the outside of the hand (propodite). The small 

 grey or purple C. rugosus is very variable (Bouvier- enumerates several varieties), but may 

 be easily distinguished from the larger, scarlet, G. perlatus by the greater development in 

 the latter of the genital process. C compressus is recognisable by its small size, grey, or 

 purple colour and less hairy integument from C. spinosus, which is larger, of a brown colour, 

 and considerably hairy in parts. The last revision — and the one adopted in this paper — 

 is to be found in Ortmann's well-known work on the Decapods of the Strassburg Museum^. 

 It differs in several points from that of Bouvier'', but has the merits of simplicity and 

 clearness, and has satisfactorily accounted, so far, for every one out of the many specimens 

 that have passed through my hands. 



The distribution of the genus would appear, from the data in Ortmann's paper and 

 others since published, to be as follows : — C. diogenes is found in the West Indian region 

 alone, and is the only representative of the genus there. The other species are all re- 

 stricted to the Indo-Pacific area, but are there of almost universal distribution, C. rugosus 

 being perhaps the most widespread. 



' See footnote to p. 69 above. 



= Bouvier, Bull. Soc. Philem., Paris, (8), ii. p. 143 (1890). 

 G. 



'■> Ortmann, Zool. Jahrb. Syst., vi. p. 315 (1892). 

 ' Bouvier, loc. cit. 



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