154 CRUSTACEANS. 



The external integument is an indurated shelly covering, composed 

 of numerous separate pieces, connected by a kind of cartilaginous sub- 

 stance, and all the limbs are composed of articulations. Perhaps this 

 external integument is strengthened by successive invisible internal secre- 

 tions, for younger animals are easily crushed, whereas it is difficult to 

 make any impression on old ones. But it is so far inorganic that it seems 

 to undergo no visible alteration during the time it serves the animal in- 

 vested. However, this is a subject which has never undergone sufficient 

 scrutiny. 



The Cancer mcenas occurs of all different dimensions, from young of 

 the smallest size to the mature adult. But I am not aware that any 

 single specimen has undergone sensible increment under observation of the 

 naturalist. If he inspects such a specimen month after month he will 

 probably find it no larger at the end than at the beginning, farther than 

 some slight relaxation of the cartilages, if that actually takes place. 

 Therefore, without the important event, to which we shall speedily ad- 

 vert, it would remain always the same. 



Yet nature may not be idle in the intervals. Secretions may be 

 advancing, though unseen, preparatory with what is indispensable for the 

 concomitant subject's preservation, and for admitting with age the in- 

 crement of an animal whose integuments are inflexible. 



At certain seasons of the year all the limbs are full of flesh, as is 

 most evident in the claws, such as of the Cancer pagurus, where they are 

 of considerable size. But there, as well as in the Cancer mcenas, while the 

 time approaches, that progressive increment shall render the capacity of 

 the subsisting shell insufficient for its contents, the limbs seem to be 

 deserted by the flesh. A soft integument is forming on the inner 

 surface of the carapace, interposing between it and the internal parts, 

 wherein these parts are about to be included. By a wonderful and in- 

 explicable operation of nature a complete new integument, ultimately 

 crustaceous, is generated within the subsisting shell, with the whole limbs 

 folded over the breast, so that the least possible portion of the internal 

 space, soon to become an absolute vacuity, is occupied. 



Next, when all is mature, the subsisting shell opens or gapes hori- 



