154 THE MORPHOLOGY OF THE COMMON CRAYFISH. 
rudimentary branchia. These aree of the epimera, in 
fact, correspond with the shield-shaped plate of the 
hindermost somite. In the next most antevior somite 
(that which bears the first pair of ambulatory legs) there 
is only a small elevation in the place of the rudimentary 
branchia ; and in the anterior four thoracic somites no- 
thing of the kind is visible. 
On the sternal aspect of the thorax (figs. 8 and 39, A) a 
triangular space is interposed between the basal joints or 
coxopodites of the penultimate and the ante-penultimate 
pairs of ambulatory legs, while the coxopodites of the 
more anterior limbs are closely approximated. The 
triangular area in question is occupied by two sterna 
(fig. 39, A, XIZ, XII), the lateral margins of which are 
raised into flange-like ridges. The next two sterna (X, 
XI) are longer, especially that which lies between the 
forceps (X), but they are very narrow; while the lateral 
processes are reduced to mere tubercles at the posterior 
ends of the sterna. Between the three pairs of maxil- 
lipedes, the sterna (VII, VIII, IX) are yet narrower, and 
become gradually shorter; but traces of the tubercles at 
their posterior ends are still discernible. The most 
anterior of these sternal rods passes into a transversely 
elongated plate, shaped like a broad arrow (V, VJ), 
which is constituted by the conjoined sterna of the two 
posterior somites of the head. 
Anteriorly to this, and between it and the posterior 
end of the elongated oral aperture, the sternal region is 
