THE YOUNG OF THE CEAYFISHES ASTACUS AND CAMBAKUS 



31 



stage there were only simple spines to anticipate some of these plumes. The 

 three large segments of the exopodite are more angnlar than before and now 

 bear a few set;B while the filament has some fifty-four segments of the form 

 and proportions shown in figure 26. As these bear needle-like set* on their dis- 

 tal edges the filament seems under a low magnification somewhat like a brush. 

 The mandible is not only greatly enlarged but more complex in having an 

 effective cutting edge no longer smooth but serrated by seven unequal angular 

 projections (fig. 27). The palp is more complex in having more numerous long, 

 acicular seta> both on the exterior and interior faces of its terminal seg-ment 

 and a very few sparsely-branched plumes on the distal part of its second seg- 

 ment. When folded down the palp fits into a deep depression on the inner 

 face of the mandible (fig. 28), and the proximal border of this depression is 

 irregularly dentated with rounded protuberances. The exbskeleton over these 

 dentations and over the sharp teeth of the cutting edge is now very thick and 

 horny, being solid as far back as the second line in figure 28. Wliile the 

 acicular setie over the terminal segment of the palp appear smooth under Zeiss 

 2 A, they are really set with short, fine, side branches along their distal halves 

 as seen with 2 D, and they have rather blunt i^oints so that they would seem to 

 aid in a brush-like use of the palp. 



The first maxilla (fig. 29) has progressed beyond its former stage (fig. 9), 

 chiefly in the outgrowth of long seta? in place of blunt spines and also in the 

 addition of set* where there were no outgrowths at all. The seta; are of two 

 kinds, a few plumose and many acicular; the latter are found chiefly on the ends 

 of the two plates of the protopodite where they replace simple spines, while the 

 plumes are chiefly lateral. On the basal, or first segment, however, there is some- 

 what of a transition, since its proximal border is set with sets that extend out 

 to the tip as very long and sparsely branched plumes that thus extend close up 

 to the acicular set^e. These latter under 4 D are seen to be set with very few 

 and fine side branches so that they are really somewhat plumose. The similarly 

 placed acicular seta^ upon the second segment, however, show no side branches 

 but are really smooth. The small tuft of seta^ already present in the first 

 stage at the base of the endopodite remains but little changed in the second 



stage. 



The second maxilla has undergone like changes (fig. 30). The terminal 

 spines of the first stage (fig. 10) are replaced by plumose seta; and a few more 

 long plumes are added. Here again the plumose seta; of the proximal edge of 

 the first segment extend out as far as the acicular seta; of the apex. The 

 endopodite has a few long plumose seta; on its distal part in addition to the 

 cluster at its base. The scaphoguathite is but little changed, the seta; along its 

 outer edge being so bent down that they do not show their full length in the 

 view represented in figure 30. However, at the posterior tip of this respiratory 



