THE CLASSIFICATION OF CRUSTACEA 63 



The first tribe of the Brachyura, the Dromiacea, 

 comprises a number of Crabs that in many points of 

 structure resemble the Lobsters, and are regarded as 

 the most primitive members of the section. Dromia 

 vulgaris (Plate IX.), a furry, clumsy-looking Crab, 

 occasionally found on our southern coasts, has the 

 last two pairs of legs short and carried up over the 

 back, where they are used for holding a mass of 

 living sponge which the Crab uses as a cloak to pro- 

 tect and conceal itself. At the sides of the abdomen, 

 wedged in between the telson and the last somite, 

 a pair of small plates may be seen, which are the 

 last vestiges of the uropods. These are wanting in 

 the other tribes of the Brachyura. 



The Oxystomata (Plate X.), which form the second 

 tribe of the Brachyura, are distinguished by the 

 form of the mouth-frame, which is narrowed in front 

 so as to be triangular instead of square in outline. 

 The passages through which the water passes out 

 from the gills, which in other Crabs open at the 

 front corners of the mouth-frame, are carried for- 

 wards to the front of the head. The Oxystomata 

 are most abundant in tropical seas, but are repre- 

 sented on the British coasts by species of Ebalia, 

 small and compact Crabs which are not unlike 

 pebbles of the gravel among which they live. 



The remaining Crabs form the tribe Brachygnatha, 

 in which the mouth-frame and the maxillipeds that 

 close it are more or less quadrilateral in shape. 



