22 THE YOUNG OF THE CRAYFISHES ASTACUS AND CAMBARUS 



which may each end with a small, blunt spine. The third maxilliped (fig. 13) 

 exhibits the typical morphology of this organ in the adult; a somewhat two 

 jointed basal protopodite bears a large five-jointed endopodite of great size, 

 a long slender exopodite, a large epipodite and podobranch; and two arthro- 

 branchi.e arise from the region connecting the appendage to the body. 



The endopodite bears spines upon all its segments and the protopodite has 

 a couple of small spines upon its distal segment and a plumose setae upon its 

 proximal .segment. The exopodite, in contrast to that of the second maxilliped, 

 has a shorter and more slender basal segment devoid of spines, while the 

 second segment has several spines at and near its tip. The lamina .of the 

 epipodite has the same characters as in the second maxilliped, but the plumose 

 set* along its basal ridge are twice as many. The podobranch is like that of 

 the second maxilliped. Of the two arthrobranchiae the anterior one is much 

 like its homologue on the second maxilliped, while the posterior one, nearer the 

 observer in figure 13, is smaller and more simple with fewer lateral filaments. 



Coming next to the ambulatory appendages, we find the usual large chela?, 

 the two pairs of slender chelate and two pairs of non-chelate legs (figs. 14, 15, 

 16, 17, 18). In these appendages of the first larva there are, as in the adults, 

 no exopodites, and even the remarkable exopodite setae of later larvas and of 

 adults are absent in this first stage, thus adding to the simplicity of the limbs, 

 which is also expressed in the entire absence of plumose sets and the presence 

 of but few sharp spines. 



The chela (fig. 14) has the recurved terminal hooks first made out by Hux- 

 ley in the English Astacus, and which lead to the firm locking of the chelae to 

 the egg stalk, as above narrated; and the opposing edges of the claw are ser- 

 rated from the presence of sharp spines pointing toward its tip. The chela 

 bears very large sharp protuberances along the inner edge of .the meropodite 

 segment, of no apparent use, while the great length and thickness of the whole 

 limb is apparently necessary in that firm holding of the larva to the mother 

 which resists the force of the maternal pleopods that swing the larvae back and 

 forth. 



The epipodite and gills of the chela; are like those of the third maxilliped. 

 The following two legs (figs. 15, 16) are like one another in every way 

 except in proportion, the first being shorter than the second. Each has a 

 sharp claw with spines pointing toward the tip, but the tips are not recurved 

 as is the case in the big chelae. The gills on these two appendages are like those 

 of the chela, but there is in addition a slender simple gill upon the body wall 

 near the arthrobranchiae. This pleurobranchia is a single filament with no 

 lateral outgrowths and may be regarded as rudimentary at this stage, as it is 

 also in the adult. 



The remaining legs (figs. 1.7, 18) have terminal segments almost like those 



