THE YOUNG OF THE CRAYFISHES ASTACUS AND CAMBABUS 45 



While the form of the telson is thus so different from that of Astacus (fig. 

 20), the mode of attachment of the telson thread is fundamentally the same. 

 Figure 48 shows the tip of the telson of a larva torn out of its shell just before 

 hatching ; the thread is fastened to five or six spines on each side of the median 

 plane in the same way as in Astacus (fig. 21). Thus nearly the same number of 

 spines are specialized in both crayfish for attachment of the thread though in 

 Cambarus affinis the entire number of spines is much less and they are found 

 only on the posterior part of the edge of the telson. 



All the spines are glass clear, ice-like in refraction; the lateral ones are 

 bent toward the tip of the telson and the five or six specialized glandular spines 

 converge toward a center as shown in figure 48. The lateral spines toward the 

 posterior end tend to show blunt brushes and secreted lobes at their tips, and 

 thus form somewhat of a transition to the effective specialized spines. 



These specialized central spines are much longer and thicker and bent, 

 often at right angles. Some are fused together by their blunt ends and all 

 seem to have flowed out at the tips as a mass which is now fibrous and which 

 binds all of them to each other and to the telson thread. They seem compar- 

 able to paste tubes which should squeeze out a myelin-like substance that could 

 coagulate as strong fibrils. 



The appearances suggested that the rows of gland cells that later make the 

 plumose setae of the later larva, had previously, in late embryonic life, secreted a 

 substance which oozed out of the hollow spines and set into a firm cement; but 

 these cells no doubt were also active in making the cuticular walls of the spines 

 themselves, and no sharp line seemed drawn between the substance of the 

 cuticular spines and the material that issued out of their tips. Both are pre- 

 sumably the same exoskeleton and made from ectoderm cells that later make 

 other exoskeletal secretions in the form of plumose setae. In Astacus (fig. 21) 

 the distinction between spine and secretion was more evident, but in this Cam- 

 barus (fig. 48) the spines are so minute that details are not as readily seen. 



While the mode of attachment of the telson thread to the telson is thus the 

 same in Astacus and in Cambarus, the thread itself differs in appearance in 

 the two crayfish, in the former being pulled out into a long thread, in the 

 latter being for the most part a wrinkled mass of membrane within the egg 

 case. While in Astacus the thread is apparently a cast off embryonic skin, this 

 is by no means obvious in Cambarus and an interpretation of its meaning was 

 had only from the following facts. When an egg ready to hatch was scratched 

 with a needle the outer egg case came off and the larva popped out alive but 

 still enclosed in a thin spheroidal membrane. This membrane was firmly 

 fastened to the outer egg case by one small area towards which the legs con- 

 verged and which lay opposite the claws. When the egg case was pulled it re- 

 mained so fast to the membrane that both wei-e drawn out of shape rather than 



