MARINE CRUSTACEANS. 



IX. THE SPONGE-CRABS (DROMIACEA). 



By L. a. Borradaile, M.A., Lecturer in Natural Sciences at Selwyn 

 College, Cambridge. 



(With Plate XXXIII.) 



The members of the Dromiacea are separated from the other groups of Brachyura by 

 a number of characters, most of which are primitive and show that they are nearer than 

 the rest of the Crabs to the macrurous ancestors from which the tribe arose. The most 

 important of these features are : the presence of a pair of vestigial limbs on the first 

 abdominal segment of the female, and often on the sixth in both sexes', the epipodites, 

 which are often found on one or more of the pairs of legs, the number of gills, which is 

 usually large, the more or less square shape of the mouth, the large, usually free, basal joint 

 of the antennal stalk, and the often incomplete orbits. To these must be added a highly 

 characteristic, but not primitive, feature — the small size, dorsal position, and usually prehensile 

 shape of one or both of the hinder pairs of legs (PI. XXXIII). 



This conformation of the hinder legs is connected with the most characteristic peculiarity 

 in the habits of the Dromiacea. Most of them are in the habit of holding over their backs 

 masses of some sessile animal — usually a sponge, but at times an ascidian, an alcyonarian, 

 or even half a bivalve shell, on which a sea-anemone is often seated — by means of which 

 they are completely hidden'-. 



The most important part of the prehensile apparatus by which this is accomplished is 

 the last joint of the legs of the hinder two pairs. This joint is sharp and hooked in most 

 genera, and is buried deeply in the substance of the sponge. There is often also a thorn 

 on the propodite, which makes wth the last joint a more or less perfect chela (PL XXXIII, 

 figs. .3 b, 4 b). In many cases, at least, the crab does not confine its choice to one species 

 of sponge. Whether the latter is at first merely a fragment broken off from a larger mass, 

 or is always a whole colony, perhaps specially adapted to this life and not found elsewhere, 

 is not known, but in any case the initiative in forming the partnership must come fi'om 



1 Bouvier (Bull. Soc. Philomath. Paris, 1896) throws doubt of Macrura. 

 on the correctuess of the view which homologises the struc- - This curious habit is also found in the Dorippidae 



tores on this segment in Dromiidae with the last pair of limbs among the Oxystomata (see above, p. 435, vol. i.). 



