70 THE YOUNG OF THE CBAYFISHES ASTACUS AND CAMBARUS 



podite while Astacus lias G in each; in having but 5 sensory hairs in place 

 of 8 in Astacus which has advanced further as is shown by the arrangement of 

 these hairs, and in having the ear pit still bare while in Astacus it is guarded 

 by a fringe of plumose seta? along its edge. In Cambarus the second antenna 

 has 39 segments and its scale has a row of a few spines only while in Astacus 

 there are 54 segments and the scale is fringed with plumose seta;. In Cambarus 

 the maxillae and maxillipeds have but few setae and no plumose ones except on 

 the tip of the last maxilliped and the edge of the scaphognathite, while in Astacus 

 the plumes are conspicuous and show a tendency to extend out to the cutting 

 edges of the protopodites and to thus replace the simple setae by highly special- 

 ized forms of setae with lateral barbs. The scaphognathite in Cambarus is 

 more simple in lacking the peculiar long plume-like setae at its posterior end. 

 The chelae are used by Cambarus to hold fast to the mother and their tips are 

 somewhat less straight than in Astacus. In Cambarus also the remarkably long 

 exopoditic setae of Astacus are not developed. 



The first two stages in which the larvae of these crayfish differ most are 

 also the stages in which crayfish are peculiar in having an intimate association 

 of larvae with parent. From the generally accepted standpoint that crayfish 

 have been evolved from marine ancestors which had pelagic, or at least, active 

 larva? that left the mother as soon as they were hatched, the association in the 

 crayfish is a new acquisition. The differences between the larvae are also pre- 

 sumably of recent origin since they make the early stages more unlike than 

 later stages, while if there were a common ancestor the greater divergence 

 would be in the most remote descendants and stages, until some newer change 

 modified the early larvae and thus "falsified" the record. 



Some of the new modifications in structure that go along with the new 

 habits of association of larvae and mother may be recognized as distinct from 

 the small size and other characters that might belong to active larva?. The 

 characters in the first and second stages that seem inimical to free larval life 

 and to be adapted to the parasite-like life upon the mother are; partly those 

 negative traits which an embryo might have in protected sedentary life and 

 which here may be in fact but the lingering on outside the egg of conditions 

 formerly found only within the egg; and partly directly adapted additional 

 characters that are peculiar to this life of larval dependence upon the pleopods 

 of the mother, and of no use elsewhere. 



The embryo-like characters of the first larval stage are as follows. The 

 spheroidal head-thorax and accompanying position of limbs and of abdomen 

 that make walking and true swimming impossible and force the creature to lie 

 upon its side if removed from its natural attachment to the mother. The weak, 

 imperfect state of the locomotor organs ; slender legs, simple telson, absence 

 of utilizable sixth pleopods. The lack of useful defensive organs, that is the 



