700 CRUSTACEA. 



as the growth is then most rapid ; as many There are some species, such as the Crabs 

 as eight changes of the tegumentary envelope and the Brachyura generally, in which the 

 have been observed to take place in the course carapace presents a considerable expansion on 

 of seventeen days in the young Daphnia; whilst either side, forming two large compartments 

 in adult Crustacea the change is not in general in which the greater mass of the thoracic vis- 

 effected oftener than once a year. cera is contained. Under these circumstances 

 Reaumur watched the phenomenon through it would be impossible for the animal to escape 

 its whole course, and has noted it with all its from its dorsal covering by the relatively in- 

 details as it occurs in the Astacus fluviatilis* considerable opening which this part presents 

 It takes place in this species towards the end on its inferior aspect. This renders it neces- 

 of summer or beginning of autumn. A few sary that the carapace, instead of being cast 

 days of fasting and sickness precede it, during off by simply rising in a single piece, 

 which the carapace becomes loosened from the should give way and separate in some direction 

 corium to which it adhered, and which im- or another, and this it does by splitting along 

 mediately begins to secrete a new one, soft the curved lines, extending on either side from 

 and membranous at first, but soon becoming the mouth to the origin of the abdomen, in the 

 harder and harder, and finally completely cal- course of which the epimeral pieces cohere 

 careous. In this way the animal before long with the dorsal one.* 



finds itself free from all connexion with its The time occupied in the business of throw- 

 old envelope, and it has only to make its ing off the shell varies considerably in dif- 

 escape. This last operation is announced by ferent species ; it is also dependent on at- 

 symptoms of inquietude. The creature rubs mospheric influences. It is the same also, 

 its legs one against another, and then throwing in regard to the number of days necessary to 

 itself upon its back begins to shake itself, give to the new epidermic layer the consistency 

 puffs itself out, so as to tear the membrane of the old tegumentary covering. A general 

 which connects the carapace with the abdo- remark, however, and one which is applicable 

 men, and to raise the carapace itself. After to the whole of the species that have been 

 sundry intervals of rest and agitation of longer duly observed, especially those that are found 

 or shorter duration, the carapace is raised com- along the shores of France, is this, that the 

 pletely ; the animal extricates its head, its eyes, period which precedes as well as that which 

 and its antennae. The operation of freeing follows the change of the shell is one of rest- 

 its extremities appears to be the most difficult, lessness and evident illness. The muscles of 

 and would even be impossible did not the these creatures are then flaccid, the flesh is soft 

 solid covering of these parts split longitudi- and watery,' and as food they are rejected as 

 nally ; but in spite of every assistance, it not tasteless and held unwholesome. This would 

 unfrequently happens that the animal leaves not appear to be the case with the Land-crab, 

 one or two of its limbs impacted within the however, according to the statements of several 

 old sheath, and occasionally even perishes travellers, who inform us that the flesh of this 

 through inability to escape completely from its species is never in greater perfection than during 

 shell. The abdomen is the last division of the season of the moult. 



the body which clears itself of the old en ve- A phenomenon, which has some analogy with 

 lope. All the parts of the tegumentary ske- the renovation of the tegumentary skeleton, 

 leton which had only been separated from one but which is much more curious, is the repro- 

 another, without however having undergone duction of the legs of these animals. Most 

 any softening, or fracture, or separation, fall Crustacea cast off their claws very easily and 

 one upon another in resuming their old posi- without apparent pain ; the separation always 

 tions, so as to represent the complete external takes place in a determinate point near the 

 form of the creature with the whole of its basis of the member (in the second articula- 

 solid internal as well as external parts ; even tion), and is soon followed by the formation 

 the eyes, the antennae, and the thoracic cells of a cicatrice, from the surface of which sprouts 

 formed by the sternal and epimeral apodemata, out a small cylindrical appendage ; this shortly 

 may be distinguished. The operation now after presents distinct articulations, and re- 

 described does not in general occupy more sembles in miniature the organ it is destined 

 than half an hour in the performance ; and to form, but its growth is slow, and it does not 

 only two or three days, or even no more than for some lime attain its full size. If one of 

 four-and-twenty hours are required to convert the limbs be severed in any other part, the 

 the soft and membranous envelope with wound continues to bleed, and no renovating 

 which the corium or naked body of the process begins unless the animal, by a violent 

 animal is surrounded, into a firm calcareous muscular contraction, succeeds in breaking off 

 covering similar to the one which has just been the stump in the articulation above mentioned, 

 got rid of. The new envelope presents the The kind of solid sheath formed by the 

 same appendages as the former one, even the tegumentary skeleton of the Crustacea, and 

 same hairs ; but these, instead of being con- which includes in its interior the whole of 

 tained within the old ones, as Reaumur ima- the viscera and other soft parts of these ani- 

 gined, exist ready formed in the new envelope, mals required to be so constructed as not to 

 but turned in towards the interior, like the oppose locomotion \ consequently there exist, 

 fingers of a glove turned in upon themselves. 



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



* Memoii'cs de FAcademie dcs Sciences, 1718. Nat. des Crustaces, t. i. p. 56. 



