48 THE YOUNG OF THE CRAYFISHES ASTACUS AND CAMBARUS 



lu neither the dorsal nor the side view (figs. -1-9, 50) can the rostrum be well seen, 

 but in a front view (fig. 51 ) it is seen to be ])ointed, though so bent downward be- 

 tween the eyes as to be of no use as a protection. 



The body is thus rounded and embryonic in proportion and lacks all the 

 angularities of adults or of active larvte. 



With the clinging helpless state of the larvjeis associated a lack of ordinary 

 use of the antenna?. The second antenna^, which in larval and adult life of Crus- 

 tacea are carried out in front of the animal, are here folded down under the 

 thorax (fig. 50), between the right and left series of legs. This peculiar posi- 

 tion of the antenna? is not found in Astacus and here is brought about at the 

 time of hatching, for in the embryo these antenna? grow liackward along the 

 edge of the carapace external to the bases of the limbs. Soon after hatching the 

 legs get astride the antenna? and this apparently useless position of the antennse 

 is maintained during the rest of the life of the larva in its first stage. 



This peculiar position of the antennne as well as the down bent rostrum 

 were observed in another species of Cambarus, C. nisticus by Faxon ('85); 

 and later Steele (:02) described the rostrum as bent down in C. gracilis and 

 in other species and the legs, antenna and abdomen of C. virilis as lying under 

 the thorax; thus it seems possible that the fii'st stage of Cambarus in general 

 differs from Astacus in these characteristics of the antenna. 



The appendages of C. affinis in this first stage resemble those of Astacus 

 leniusculus in being without the setiB of later stages, but they differ not only in 

 being very much smaller but in being, in some cases, more simple. Thus the 

 first antenna (fig. 53) has only four segments in the endopodite and in the ex- 

 opodite in place of five, and the tip of the endopodite bears no sensory club. 



The antenna (fig. 54) is remarkably short, as is seen on comparing that 

 figure with figure 7, and figure 50 with figure 3. The filament is bent back and 

 has but twenty-five segments, or about half as many as Astacus. In the adult 

 there may be 150 segments. Carried as it is imder the thorax it reaches only 

 to the beginning of the abdomen while in Astacus the antenna if in such a posi- 

 tion would reach about to the end of the abdomen when stretched out. 



The mandible (fig. 55) has the same simple form as in Astacus (fig. 8). 



The first maxilla (fig. 56) is like that of Astacus (fig. 9), but more simple 

 in lacking the few filose seta? and in having fewer spines. 



The second maxilla (fig. 57) as in Astacus (fig. 10), has a row of plumose 

 setae along the entire edge of the scaphognathite and is elsewhere as simple. 



The maxillipeds (figs. 58, 59, 60) represent in miniature the structure seen 

 in Astacus (figs. 11, 12, 13) with slight increase in smoothness due to the pres- 

 ence of fewer setae and spines. There are important simplifications, however, in 

 the gills which in some cases have Init half as many side filaments as in Astacus. 



The chelae (figs. 61, 62) have the same recurved tips as in Astacns. Soon 



