BRACHYURA 



407 



all the latter of well-formed uropods, which the true crabs do not 

 possess, and by a fusion of the edge of the carapace with the epistome, 

 a sternal plate which lies in front of the mouth (see p. 410). 



As an example of the Brachyura we shall describe Carcinus maenaSy 

 the common shore crab of Britain (Figs. 284, 285, 287-291). The 

 depression to which is due the difference in shape between the 

 cephalothorax of this typical crab and that of a crayfish or prawn has 

 brought it about that in a transverse section (Fig. 285) the carapace 



cb.sp. 



en^ 



£p< 



Fig. 285. 



Fig. 286. 



Fig. 285. Diagram of a transverse section through a branchial chamber of 

 Carcinus maenas. From Borradaile. a.l.e. anterolateral edge; cp. coxopodite; 

 eb.sp. epibranchial space; ep.i, mastigobranch (distal part of epipodite) of 

 the first maxilliped; ep.^, mastigobranch of the third maxilliped; hy. hypo- 

 branchial space ; i.r. inner layer of branchiostegal fold ; o.r. outer layer of the 

 same; proc. process of flank of thorax, which meets branchiostegite and 

 separates two of the openings above the legs into the chamber. 



Fig. 286. The left third maxilliped of Astacus. cp.set. coxopoditic setae; 

 en. endopodite; ep. epipodite; ex. exopodite. 



has at the sides (where, as the branchiostegite, it covers the gills), not 

 an arched profile but runs out almost horizontally and is then bent 

 in, at an angle {a.l.e.) which is more acute in the anterior part of the 

 body than in the hinder part, to end against the flank above the coxo- 

 podites of the legs. At the angle, the branchiostegite, viewed from 

 above, describes the lateral part of the outline of the body. That out- 

 line begins between the eyes, where in the crayfish the rostrum stands, 

 with the/row^, a low, three-toothed lobe. On each side of this is the 

 orbity an excavation of the surface of the head for the reception of the 



