EXUVIATION OF ASTACUS FLUVIATILIS. 393 



thoroughly every part of its worn-out covering from all connexion with 

 the recently-secreted investment. This being accomplished, it remains 

 to extricate itself from its imprisonment an operation of some diffi- 

 culty ; and when the nature of the armour to he removed is considered, 

 we may well conceive that not a little exertion will be required before 

 its completion. As soon as the old case of the cephalothorax has be- 

 come quite detached from the cutis by the interposition of the newly- 

 formed epidermic layer, it is thrown off in one piece, after great and 

 violent exertion ; the legs are then withdrawn from their cases, also after 

 much struggling ; and, to complete the process, the tail is ultimately, 

 by long-continued efforts, extricated from its calcareous covering, and 

 the entire coat of mail which previously defended the body is discarded 

 and left upon the sand. The phenomena which attend this renovation 

 of the external skeleton are so unimaginable, that it is really extra- 

 ordinary how little has been done towards elucidating the nature of 

 the operation. The first question which presents itself is, how are the 

 limbs liberated from their confinement ? for, wonderful as it may appear, 

 the joints even of the massive chelae, of the Lobster do not separate from 

 each other, but, notwithstanding the great size of some of the segments 

 of the claw, and the slender dimensions of the joints that connect the 

 different pieces, the cast-off skeleton of the limb presents exactly the 

 same appearance as if it still encased the living member. The only way 

 of explaining the circumstance is to suppose that the individual pieces 

 of the skeleton, as well as the soft articulations connecting them, split in 

 a longitudinal direction, and that, after the abstraction of the limb, the 

 fissured parts close again with so much accuracy that even the traces 

 of the division are imperceptible. But this is not the only part of the 

 process which is calculated to excite our astonishment: the internal 

 calcareous septa from which the muscles derive their origins, and the 

 tendons whereby they are inserted into the moveable portions of the 

 outer shell, are likewise stated to be found attached to the exuviae ; 

 even the singular dental apparatus situated in the stomach, of which 

 we shall speak hereafter, is cast off and re-formed ! And yet, how is 

 all this accomplished ? how do such parts become detached ? how are 

 they renewed ? We apprehend that more puzzling questions than these 

 can scarcely be propounded to the physiologist ; nor could more inter- 

 esting subjects of inquiry be pointed out to those whose opportunities 

 enable them to prosecute researches connected with their elucidation*. 



* Since writing the above, I have been fortunate in procuring a very good speci- 

 men of Astacus fluviatilis, obtained soon after casting its shell, and also its newly 

 cast-off covering, both of which are in excellent preservation. The following is a 

 description of the appearances observed in each : All the pieces of the exuvise were 

 connected together by the old articulations, and accurately represented the external 

 form of the complete animal, the carapace, or dorsal shield of the cephalothorax, 

 alone being detached, having been thrown off in one piece. The pedicles of the eyes 

 and external corneas, as well as the antennae, remaind in situ, the corresponding parts 



