74 THE YOUNG OF THE CKAYFISHES ASTACUS AND CAMBAKUS 



second antenna is less perfect in having only 39 in place of 54 segments and in 

 still having only spines in place of iiliinies iipon its scale. In Cambarus also 

 the second stage is notably lacking in almost all plumose setae and is thus 

 further removed from fitness for free active life. 



Tile third stage of Cambarus though rivaling Astaeus in perfection of loco- 

 motor and sensory apparatus still associates longer with the mother and thus 

 suggests a future period when the larval dei)endence may be extended so that 

 more than two larval stages may remain upon the mother. 



Though the young of Cambarus are better fitted to stay with the mother, 

 and do so for a longer ])eriod of their life as reckoned in stages, the actual 

 number of days in which the association continues is not essentially different in 

 Cambarus and in Astaeus. In the former the larvaj is fixed on the mother for 

 a week and then remains in association for another week or more. In the latter 

 the larva is fixed to the mother four to fourteen days and then is associated for 

 only a few days more, as far as was made out in laboratory culture of A. lcn- 

 iuscidus, while Chantrau ('70) states the young of a European species are fixed 

 for ten days and then go and come for ten days more. 



The conclusion that Cambarus has advanced further than Astaeus in the 

 adaptation of its youug to a life of association with the mother is in harmony 

 with the relative positions of these two genera as determined by anatomy and 

 geographical distribution. The absence of the pleurobranchiae in Cambarus and 

 the presence of a specialized sperm receptacle, the so-called annulus ventralis, 

 as well as the corresponding specialization of the male stylets, are some of the 

 important characters that show the adult Cambarus to be more highly evolved 

 than Astaeus, and Ortmann ( :02) has shown how the evolution of all the species 

 of Cambarus may have taken place since divergence from Astaeus like ances- 

 tors in the region of Mexico. 



We may therefore suppose that as Cambarus has migrated over the middle 

 and eastern United States it has become s[ilit up into the sixty odd species now 

 found and in some as in C. (iffiiiis has made more perfect the association of 

 young and parent already i)resent in the Astaeus ancestor. 



As all the crayfish are primarily fresh water or land dwellers it is natural 

 to seek to connect their possession of an elementary family state with their de- 

 parture from their ancestral marine life and it is easy to imagine that the species 

 might be benefited by the young being protected by the mother when either liv- 

 ing in holes or moving from i)lace to place till of larger size and more perfect 

 structure. But it is not obvious that the advantage would be greater in fresh 

 water than in salt water life, and as we know the lobster has already greatly 

 shortened its ancestral series of metamorphoses and that the crayfish family- 

 life is possil)le only after such shortening of metamorphoses, we seem, in the 

 lack of evidence as to when the association found in the crayfish really began, 



