THE YOUNG OF THE CRAYFISHES ASTACUS AND CAMBARUS 53 



for a certain distance up from the anus, delays in casting off its cuticle. The 

 animal bursting out of its head-thorax cuticle and freeing its limbs from their 

 cases would next pull its abdomen out of its old cuticle, but the old cuticular 

 lining of the intestine remains firm where continuous with the outside cuticle at 

 the edges of the anus, and acts like a string tied to the old telson and to the inside 

 of the abdomen far up in the intestine. If the larva then drops out of its cuticle 

 and is too weak to seize hold of the old cuticle, or neighboring firm objects, it 

 will dangle suspended by the anal thread and the tension on this thread will 

 pull it out of the anus as far as possible. By puckering the posterior part of the 

 intestine and by dragging the anterior part of the intestine backward there is 

 thus drawn out of the anus an anal string which, however, is not as long as 

 the abdomen, by any means. The old telson being a stiff plate and the old 

 walls of the abdomen being elsewhere like the bellows of a camera, the pull 

 of the weight of the larva along the telson thread drags the old telson against 

 the cast-off abdominal rings and telescopes the abdominal cuticle as if the front 

 board of a camera were pulled back against the bellows by a string inside. The 

 tendency of the telson string would be to turn the abdominal cuticle of the cast- 

 off case inside out as a hand holding fast to a sleeve would turn the sleeve 

 inside out when the arm was withdrawn, but the presence of a firm, board-like 

 telson at the bottom of the sleeve allows only a telescoping of the cuticle. By 

 this means the old abdominal walls are so shut up that the free anal string 

 from the old anus to new anus is no longer inside the old abdomen, but largely 

 free in the water, as indicated in the diagram on page 54. 



Soon after hatching, presumably, the cuticular lining of the intestine be- 

 comes loosened along the regions (a) and (x) and then the anal thread is 

 pulled out from the intestine as is usual in moulting. In the meantime the larva 

 becomes strong enough to reach about and take hold of the old larval skin or 

 of other objects fastened to the pleopods and does not need the anal string as 

 a means of support. 



By this delay in the casting off of part of the cuticular lining of the in- 

 testine an advantage to the larva seems to result. Both in the first stage and in 

 the second stage the larva has a mechanical means of fixation to its mother 

 during the brief period when it is unable to use its claws for this purpose. 

 Reviewing the life of the crayfish up to the third stage with the aid of a rude 

 diagram (page 54), with reference to the association of mother and offspring, we 

 have seen : the egg fastened to the pleopod setae by a material that hardens soon 

 after the egg is laid; the larva hung for awhile by its telson thread; then 

 holding by its claws as well as telson thread till the latter breaks when claws 

 alone hold it locked to the mother; then dangled loose as a helpless second 

 stage supported by an anal thread till able to take hold with its claws and cast off 

 the thread, that is, to finish the moulting ; finally moulting again and as an active 



