170 THE MORPHOLOGY OF THE COMMON CRAYFISH. 



the limbs of the thorax are all reducible to the same type 

 as those of the abdomen, if we suppose that, in the 

 posterior five pair, the exopodites are suppressed ; and 

 that, in all but the last, podobranchise are superadded. 



Turning to the appendages of the head, the second 

 maxilla (fig. 47, C) presents a further modification of the 

 disposition of the parts seen in the first maxillipede. 

 The coxopodite (cxp) and the basipodite (hp) are still 

 thinner and more lamellar, and are subdivided by deep 

 fissures which extend from their inner edges. The 

 endopodite {en) is very small and undivided. In the 

 ^(^lace of the exopodite and the epipodite there is only 

 one great plate, the scaphognathite (sg) which either 

 is such an ej^ipodite as that of the first maxillipede 

 with its anterior basal process much enlarged, or repre- 

 sents both the exopodite and the epipodite. In the first 

 maxilla (B), the exopodite and the epipodite have dis- 

 appeared, and the endopodite (en) is insignificant and 

 unjointed. In the mandibles (A), the representative of 

 the protopodite is strong and transversely elongated. Its 

 broad inner or oral end presents a semicircular mastica- 

 tory surface divided by a deep longitudinal groove into 

 two toothed ridges. The one of these follows the con- 

 vex anterior or inferior contour of the masticatory surface, 

 projects far beyond the other, and is provided with a sharp 

 serrated edge; the other (fig. 43, a) gives rise to the straight 

 posterior or superior contour of the masticatory surface, 

 and is more obtusely tuberculated. In front, the inner 



