THE YOUNG OF THE CRAYFISHES ASTACUS AND CAMBAEUS 25 



the buds of the pleopods on either side of the intestine and anus. In structure 

 it is essentially a translucent, vascular, connective tissue mass over which are 

 scattered brilliant vermilion pigment cells, indicated in black in figure 20, and 

 which is covered by a thin epidermis and thin chitinous cuticle. Along most of 

 the free margin of the telson is a row of blunt, stiff papillse, or spines. In all 

 there are about sixty-six of these spines arranged symmetrically, half on each 

 side of the median plane. In the figure of Astacus given by Eeichenbach ('86) 

 there are but twenty-five spines or dentations on each side of the median line 

 and this may well be a character of systematic value. In addition to the 

 thirty-three lateral spines of Astacus leniusculus there were six or seven 

 smaller, blunt spines on each side which stand in between some of the larger 

 ones, one small one between two larger ones, and generally not so near the edge 

 but more up on the dorsal surface of the telson. 



The interior of the telson has a radiated appearance like that figured by 

 Bemak and by Eeichenbach and which was referred by Huxley to the disposi- 

 tion of vascular canals; but in our present larvae this radiation is due to long 

 delicate lines passing centrally inward, one from each lateral spine and from 

 some of the smaller spines, thus making the divergent system radiating from 

 near the anal region as shown in figure 20. Subsequent events show that these 

 lines are the forming plumose setse for the perfect locomotor telson of later 

 larval stages. Each when enlarged (figs. 21, 22) is a bundle of fibrillae that are 

 very small in comparison with the nuclei of the epidermis as each bundle of very 

 many fibrils is but once or twice the diameter of a nucleus. These radiating 

 lines are in fact to be likened to compressed bottle brushes which later will 

 expand as the perfect locomotor plumes on the telson of the second larval stage 

 (fig. 23). The plumes are being made within epidermal tubes or glands and 

 between the successive radiating glands the vascular spaces, also radiating, 

 form the justification for Huxley's interpretation. 



During the first hours of larval life the telson is connected to the inside of 

 the egg case by the long telson thread represented in figure 4. This is a trans- 

 lucent, chi tin-like membrane, or flat ribbon, showing a striated appearance due 

 to fine wrinkles in it (fig. 20), but otherwise apparently homogeneous. Though 

 seemingly but superficially attached to the telson the contact is a very firm one 

 so that when the larva succeeds in getting hold of the egg stalk and finally 

 flaps its abdomen strongly enough the telson thread is broken before it is torn 

 loose from the telson. When enlarged (fig. 21) the mode of attachment of the 

 very thin but tough membrance is seen to be, that ten of the marginal spines of 

 the telson bear special hook-like projections that are fast to the membrane. 

 These few spines are different from the others though some of the adjacent 

 ones have somewhat of the same structure at their tips. While in figure 21 the 

 spines and their processes are represented in black, they are in nature trans- 



8 



