154 THE MORPHOLOGY OF THE COMMON CRAYFISH. 



rudimentary branchia. These arese of the epimera, in 

 fact, correspond with the shield-shaped plate of the 

 hindermost somite. In the next most anterior 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. 3 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, XII, XIII), 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, VI), 

 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 



