THE YUUNG OF THE CRAYFISHES ASTACUS AND CAMBAKUS 



23 



of the preceding legs, but there being no opposing outgrowth of the propodite, 

 there is no claw. The legs increase in length and in slenderness from before 

 back in the series behind the chelae. 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 place 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 arthro- 

 branchs 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 branchial 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 Astacus pal- 

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

 of the first leg. 



The above illustrations of the separate appendages 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 maxillae and maxilliped of an em- 

 bryo not yet hatched as depicted by Eathke ('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 Eeichenbach ('86). 



Upon comparing the adult appendages of Astacus leniusculus with those 

 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 setae correlated with evident lack 

 of locomotion and probable weakness of sensory activity. Excepting the chela> 

 the cephalothoracic appendages had no obvious use. The first antenna, having 



