26 THE YOUNG OF THE CRAYFISHES ASTACUS AND CAMBABUS 



parent, colorless and very inconspicuous, especially the processes which are 

 hyalin, myelin-like protrusions, suggesting the products of glandular activity. 

 While many of these protrusions end bluntly others seem to be continued as 

 fine threads that are fast to the membrane or telson thread. Moreover, many 

 of the protrusions bridge over the space from one spine to the next and are con- 

 tinuous with adjacent protrusions as if they had flowed out when viscid and then 

 coagulated. Such bridges make adjacent spines into' hooks that hold fast to 

 the membrane, but the ultimate and essential fastening of the membrane to the 

 telson is by fine threads of some coagulated material that seems to be a con- 

 tinuation of the grosser protrusions figured in black in figure 21. 



It would appear from the statement of M. Eobin cited above (page 15) 

 that in the Astacus studied by Chantran there are but eight spines used for 

 attachment to the telson thread. 



The origin of these peculiar glandular spines, for such they seem to be, 

 is to be sought in the embryo. At the time of hatching the egg capsule breaks 

 and for a brief period the embryo may still be enveloped in a very delicate 

 membrane which passes over the abdomen and all about the telson. At that 

 period the spines of the terminal part of the telson abut against the invest- 

 ing membrane as shown in figure 22, which is from a specimen just hatch- 

 ing and killed in Pereny's liquor. Here are shown the epidermal nuclei, the 

 striations, or forming plumes of the future telson, continuing up through the 

 body of the spines, only three of which are drawn ; and at the tips of the spines 

 fountain-like masses of blunt protrusions, swollen at the tip and in many cases 

 pressed against the membrane. It would seem that a viscid mass had been 

 poured out from the tips of the spines and that this oozing out in threads had 

 become firmly soldered to the membrane. 



As far as made out the origin of the supporting telson thread is thus as 

 follows. The thread is really a membrane and when the embryo is hatching 

 this membrane is spread like a loose skin all over the embryo inside of the 

 egg case. When the embryo hatches it also sheds this membrane, coming 

 finally to pull its abdomen out of the part of the membrane that surrounds 

 the abdomen like a long bag. The bottom of the bag is, however, fastened to 

 the tip of the telson as indicated in figure 22, so that the creature cannot get 

 entirely free from the bag but pulling out its abdomen pulls up the bottom of 

 the bag and turns the bag inside out. The struggles of the larva drag the 

 membrane into a long thread, and one end of this remains attached to the tel- 

 son spines as seen in figure 21. Once this is accomplished the fact that the 

 clear thread is really a cast-off membrane would not be suspected, since it 

 seems a homogeneous, finely wrinkled thread that might well be a secretion. 



The similar structure in the European Astacus was mentioned merely as 

 a filament as cited above, page 15, but Huxley intuitively queried if it might 

 be a larval skin. 



