THE YOUNG OF THE CRAYFISHES ASTACUS AND CAMBAEUS 69 



COMPARISONS AND CONCLUSIONS. 



The chief differences between the young of Astacus leniusculus and those 

 of Cambarus affinis are to be found in the first and second stages and are dif- 

 ferences in size, in habit, and in structure. 



The differences above described may be enumerated as follows. In Cam- 

 barus affinis the larva coming from a small egg about 1.75 mm. in diameter 

 is about 4 mm. long and lives only two days before moulting. In Astacus lenius- 

 culus the larva coming from a very large egg, 2.5 mm. in diameter, is about 

 9 mm. long and lives four or more days before moulting. The first larval stage 

 in Cambarus has a simplified telson not well armed but having only 26 spines 

 which are upon its posterior edge only, while the Astacus has 66 spines that are 

 set along the lateral as well as the posterior edges. The abdominal append- 

 ages differ in that the first pair are absent in Astacus while barely recogniz- 

 able in Cambarus and the four following have equal exopodites and endopodites 

 in Cambarus while in Astacus the exopodite is longer. The telson thread is 

 short in Cambarus and long in Astacus, and in Cambarus there seems no part of 

 it a recent cuticle cast off by the embryo with well developed limbs as is the case 

 in Astacus. The first antenna in Cambarus is more simple in having only four 

 in place of five segments in its exopodite and in its endopodite and in lacking 

 the sense-hair of Astacus. The second antenna in Cambarus has 25 segments 

 while Astacus has 50; moreover, Cambarus carries this antenna bent backward 

 close to the body between the legs while Astacus carries it forward, but de- 

 pressed. In Cambarus the first maxilla lacks the few plumose setae of Astacus 

 and has fewer spines; and the maxillipeds have fewer setae and spines, as well 

 as gills of more simple structure. The gill formula of Cambarus is already 

 that of the adult and is therefore more simple than that of Astacus, but besides 

 this generic difference the gills of Cambarus are more simple in structure, in 

 this first larval stage, in many cases, especially the anterior arthrobranchs of 

 the pereiopods. 



In the second larval stage the habits of the two are different in that the 

 young Cambarus still remain inactively fixed upon the mother and are aided in 

 doing so by an anal thread, while the young of Astacus soon wander away from 

 the mother. The former are about 5 mm. long and live about six days before 

 moulting while the latter are 11 mm. long and live 8 to 10 days before moult- 

 ing. In Cambarus the rostrum is more bent and not so efficient as a protection 

 and the setae over the entire animal are much less developed. The abdomen of 

 Cambarus has only weak spines upon its telson and these are along its pos- 

 terior edge only, while in the active Astacus there is a complete fringe of long 

 plumose setae on all the edges of the telson. In Cambarus the first antenna 

 is less perfected ; in having only 5 segments in its exopodite and 4 in its endo- 



