THE YOUNG OF THE CRAYFISHES ASTACUS AND CAMBARUS 



23 



oi' the preceding legs, but there being no opposing outgrowth of the proi)odite, 

 tliere is no claw. The legs increase in length and in sleuderness from before 

 back in the series behind the chehp. The penultimate leg (fig. 17) has a longer 

 pleurobranch associated with it but otherwise its gills are as in the preceding 

 limb. The last leg, however (fig. 18), has its gills suddenly reduced; in i)lace of 

 the epipodite and podobranch there are but a few plumose hairs such as stand 

 upon the basal ridge of the epipodite of the preceding somites. The artliro- 

 bi-anclis are entirely absent and there is but one gill which is a pleurobranch, 

 which, however, in place of being a simple filament or rudiment, resembles a re- 

 duced or simple arthrobranch in that it has about seven short lateral processes 

 in two imperfect rows. 



The l)ranchial formula of the first larval stage of Astacus leniusculus is 

 then as is given in the table below. This was found to be just the same in the 

 adult of this species and it is said to be the same in the English Asfdfiis pal- 

 lipes, except that the latter lacks the rudimentary pleurobranch on the somite 

 of the first leg. 



The above illustrations of the separate apjiendages of the head- thorax are 

 the first ones as yet given of any larval Astacus, since the previous illustra- 

 tions of European forms are only the small maxillap and maxilliped of an em- 

 bryo not yet hatched as depicted by Rathke ('29, fig. 29), the tip of the chela 

 shown by Huxley ('80, fig. 8), and the under side of the abdomen with its 

 pleopods figured by Reichenbach ('86). 



Upon comparing the adult ap]iendages of A.sfdcus leniusculus witli tliose 

 of first larva as above described, the fundamental agreement in morphology 

 was obvious, but there were the following differences which all suggest a linger- 

 ing on of embryonic characters into the life outside the egg-shell. Throughout 

 all the appendages there was a marked lack of setne correlated with evident lack 

 of locojnotion and in-obable weakness of sensory activity. Excepting the chel« 

 file cephalofhoracic appendages had -no obvious tisc The first antenna, having 



