700 



CRUSTACEA. 



as the growth is then most rapid; as many 

 as eight changes of the tegumentary envelope 

 have been observed to take place in the course 

 of seventeen days in the young Daphnia; whilst 

 in adult Crustacea the change is not in general 

 effected oftener than once a year. 



Reaumur watched the phenomenon through 

 its whole course, and has noted it with all its 

 details as it occurs in the Astacus fluviatilis* 

 It takes place in this species towards the end 

 of summer or beginning of autumn. A few 

 days of fasting and sickness precede it, during 

 which the carapace becomes loosened from the 

 corium to which it adhered, and which im- 

 mediately begins to secrete a new one, soft 

 and membranous at first, but soon becoming 

 harder and harder, and finally completely cal- 

 careous. In this way the animal before long 

 finds itself free from all connexion with its 

 old envelope, and it has only to make its 

 escape. This last operation is announced by 

 symptoms of inquietude. The creature rubs 

 its legs one against another, and then throwing 

 itself upon its back begins to shake itself, 

 puffs itself out, so as to tear the membrane 

 which connects the carapace with the abdo- 

 men, and to raise the carapace itself. After 

 sundry intervals of rest and agitation of longer 

 or shorter duration, the carapace is raised com- 

 pletely ; the animal extricates its head, its eyes, 

 and its antennae. The operation of freeing 

 its extremities appears to be the most difficult, 

 and would even be impossible did not the 

 solid covering of these parts split longitudi- 

 nally; but in spite of every assistance, it not 

 unfrequently happens that the animal leaves 

 one or two of its limbs impacted within the 

 old sheath, and occasionally even perishes 

 through inability to escape completely from its 

 shell. The abdomen is the last division of 

 the body which clears itself of the old enve- 

 lope. All the parts of the tegumentary ske- 

 leton which had only been separated from one 

 another, without however having undergone 

 any softening, or fracture, or separation, fall 

 one upon another in resuming their old posi- 

 tions, so as to represent the complete external 

 form of the creature with the whole of its 

 solid internal as well as external parts ; even 

 the eyes, the antennae, and the thoracic cells 

 formed by the sternal and epimeral apodemata, 

 may be distinguished. The operation now 

 described does not in general occupy more 

 than half an hour in the performance ; and 

 only two or three days, or even no more than 

 four-and-twenty hours are required to convert 

 the soft and membranous envelope with 

 which the corium or naked body of the 

 animal is surrounded, into a firm calcareous 

 covering similar to the one which has just been 

 got rid of. The new envelope presents the 

 same appendages as the former one, even the 

 same hairs ; but these, instead of being con- 

 tained within the old ones, as Reaumur ima- 

 gined, exist ready formed in the new envelope, 

 but turned in towards the interior, like the 

 fingers of a glove turned in upon themselves. 



* Memoires de I'Academie des Sciences, 1718. 



There are some species, such as the Crabs 

 and the Brachyura generally, in which the 

 carapace presents a considerable expansion on 

 either side, forming two large compartments 

 in which the greater mass of the thoracic vis- 

 cera is contained. Under these circumstances 

 it would be impossible for the animal to escape 

 from its dorsal covering by the relatively in- 

 considerable opening which this part presents 

 on its inferior aspect. This renders it neces- 

 sary that the carapace, instead of being cast 

 off by simply rising in a single piece, 

 should give way and separate in some direction 

 or another, and this it does by splitting along 

 the curved lines, extending on either side from 

 the mouth to the origin of the abdomen, in the 

 course of which the epimeral pieces cohere 

 with the dorsal one.* 



The time occupied in the business of throw- 

 ing off the shell varies considerably in dif- 

 ferent species; it is also dependent on at- 

 mospheric influences. It is the same also, 

 in regard to the number of days necessary to 

 give to the new epidermic layer the consistency 

 of the old tegumentary covering. A general 

 remark, however, and one which is applicable 

 to the whole of the species that have been 

 duly observed, especially those that are found 

 along the shores of France, is this, that the 

 period which precedes as well as that which 

 follows the change of the shell is one of rest- 

 lessness and evident illness. The muscles of 

 these creatures are then flaccid, the flesh is soft 

 and watery, and as food they are rejected as 

 tasteless and held unwholesome. This would 

 not appear to be the case with the Land-crab, 

 however, according to the statements of several 

 travellers, who inform us that the flesh of this 

 species is never in greater perfection than during 

 the season of the moult. 



A phenomenon, which has some analogy with 

 the renovation of the tegumentary skeleton, 

 but which is much more curious, is the repro- 

 duction of the legs of these animals. Most 

 Crustacea cast off their claws very easily and 

 without apparent pain ; the separation always 

 takes place in a determinate point near the 

 basis of the member (in the second articula- 

 tion), and is soon followed by the formation 

 of a cicatrice, from the surface of which sprouts 

 out a small cylindrical appendage ; this shortly 

 after presents distinct articulations, and re- 

 sembles in miniature the organ it is destined 

 to form, but its growth is slow, and it does not 

 for some time attain its full size. If one of 

 the limbs be severed in any other part, the 

 wound continues to bleed, and no renovating 

 process begins unless the animal, by a violent 

 muscular contraction, succeeds in breaking off 

 the stump in the articulation above mentioned. 

 The kind of solid sheath formed by the 

 tegumentary skeleton of the Crustacea, and 

 which includes in its interior the whole of 

 the viscera and other soft parts of these ani- 

 mals required to be so constructed as not to 

 oppose locomotion; consequently there exist, 



* Collinson, Phil. Trans. 1746 and 1751 ; Hist. 

 Nat. des CrustacSs, t. i. p. 56. 



