122 COMPARATIVE ANATOMY 



podite are modified to form broad thin plates with sharp setose 

 edges. The epipodite is no longer a functional gill, but a thin 

 membranous plate entirely devoid of branchial filaments. The 

 endopodite is small and has only two joints, while the exopodite 

 is relatively large (fig. 27, F/). The second maxilla, though it 

 bears some resemblance to the first maxilliped, is very unlike any 

 of the other limbs that follow it, and its flat foliaceous appearance 

 at once recalls an abdominal limb of Apus. The coxopodite 

 and basipodite are represented by two thin lamelliform lobes, 

 each of which is deeply sub-divided by a fissure which extends 

 from its inner edge. The endopodite is small and unjointed, 

 though in shape not unlike the endopodite of the succeeding 

 limb. The exopodite is profoundly modified, being converted 

 into a broad thin anterior lobe and produced backwards to 

 form a similar posterior lobe. The whole structure has some 

 resemblance to a human ear. It is an accessory respiratory 

 organ used for baling water out of the branchial chamber, 

 and is known as the scaphognathite. Let us now see whether 

 there is anything more than a superficial resemblance between 

 this second maxilla and one of the typical limbs of Apus. The 

 maxilla consists of a median or axial piece to which lobes are 

 attached : this is itself attached by one end to the body and 

 represents the axis or corm of the limb of Apus. On its inner 

 side are the bifid coxopodite and basipodite, standing for 

 the four proximal endites. Beyond these is the endopodite, 

 standing for the fifth endite, and the sixth endite is represented 

 by the exopodite or distal lobe of the scaphognathite. The 

 proximal lobe of the scaphognathite suggests a flabellum, which 

 is fused with the sixth endite, yet this is probably not the case, 

 but the lobe simply represents a great extension of the dorsal 

 process of the sixth endite which is so conspicuous in the 

 second thoracic limb of Apus. The exites of the phyllopod 

 limb are probably represented in the crayfish by the epipodite 

 which in the second maxilla may be represented by two small 

 prominences on the outer side of the axis. 



Having once established these homologies, it is easy enough 

 to trace the connection between the other limbs of the cray- 

 fish and the phyllopod pattern. In the first maxilliped of 

 the crayfish the broad lobes of the coxopodite and basipodite 

 are shown, by comparison with the second maxilla, to repre- 

 sent the four proximal endites, and the same comparison shows 



