32 THE YOUNG OF THE CRAYFISHES ASTACUS AND CAMBARUS 



organ there is a peculiar growth of one or two setae. These are very long and 

 though appearing quite smooth and hair-like, with 2 D, they are seen to be really 

 set with short, fine side bristles, and they are sharp pointed. On the inner faces 

 of the four lobes of the protopodite there are now many long, sharp, curved, 

 setae. 



In the first maxilliped (fig. 31) the process of substituting plumes and acic- 

 ular setae for spines has been carried on in the same general way. The cut- 

 ting edges of the protopodite now bristle with setae most of which are simple, 

 some straight, some with curved tips ; but the tendency to run the plumose seta; 

 up to the tip of the basal segment is carried so far that these plumes take 

 entire possession of this basal segment and only the second segment has acic- 

 ular seta?. Even on the second segment a few plumes come up nearly to the tip, 

 on the distal side. The endopodite bears but a few plumose setae while the 

 long exopodite has in addition to the former series of plumes along the outer 

 edge of the basal part (fig. 11) several remarkably long and conspicuous 

 plumes near its tip. The terminal part of the exopodite is cylindrical and tends 

 to become segmented and it is from its last and its penultimate segments that 

 the long plumes project and form a terminal brush. The flat epipodite scale 

 has still a few minute, blunt papillae along its posterior edge (figs. 31, 11), and 

 on its outer and inner edges a few of the remarkable exoskeletal hooks char- 

 acteristic of these organs. 



The second maxilliped (fig. 32) has changed chiefly in adding plumes and 

 setae. The gills remain as before (fig. 12) but are longer. The protopodite has 

 added longer and more numerous plumose setae and a very few acicular ones. 

 The exopodite has substituted for its few terminal spines a brush of several 

 remarkably long, strong plumes (fig. 32), and for the sparse spines along the 

 outer edge of its basal segment a few setae which proximally are sparsely 

 plumed and distally smooth without barbs. The endopodite looks much changed 

 owing to the development of many long and stout acicular setae over its terminal 

 parts and inner edges. The setae of the endopodite are smooth as seen with 

 2 A, but with 2 D some five or six on the inner face of the second segment 

 and again on the distal edge of the fourth segment are finely barbed. In the 

 latter position two at the corner toward the exopodite have their fine lateral 

 branches flattened like saw teeth, so that they resemble the cleansing setae upon 

 the penultimate segment of the fifth leg of the adult, elsewhere described 

 (Andrews, '04). The fifth, or terminal segment of the endopodite, is armed 

 with long, smooth, stout, spine-like setae, with blunt points. 



The third maxilliped has added but few plumes but many very long acic- 

 ular setae (figs. 33, 13). The plumes are a noticeable bunch at the tip of the 

 exopodite and a few on its basal part, as well as very few on the protopodite 

 and an increased number at the base of the epipodite. The gill region is now 



