POWEB OF KEPEODTJCINQ CAST-OFF LIMBS. 407 



almost the semitransparency of ivory, whilst the latter has a chalky 

 opacity. In a transverse section of the claw, the tuhuli may be seen to 

 radiate from the central cavity towards the surface, so as very strongly 

 to resemble their radiation in a tooth; and the resemblance is still 

 further increased by the presence, at tolerably regular intervals, of 

 minute sinuosities corresponding with the laminations of the shell, which 

 seem, like the secondary curvatures of the dentinal tubuli, to indicate 

 successive stages in the calcification of the animal basis. This inner 

 layer rises up, through the pigmentary layer of the Crab's shell, in little 

 papillary elevations ; and it is from the deficiency of the pigmentary 

 layer at these parts that the coloured portion of the shell derives its 

 minutely speckled appearance. Many departures from this type are 

 presented by the different species of Crustacea. Thus, in the Prawns 

 there are large stellate pigment- cells, the colours of which are often in 

 remarkable conformity with those of the rock-pools frequented by these 

 creatures ; whilst in the Shrimps there is seldom any very distinct trace 

 of the cellular layer, and the calcareous portion of the skeleton is dis- 

 posed in the form of concentric rings, an approach to which arrangement 

 is seen in the papilla of the surface of the deepest layer in the Crab's 

 shell. 



(1039.) The singular power of breaking off their own limbs, alluded 

 to in paragraph 1037, is possessed by many Crustacea, and is a very 

 indispensable provision in their economy. We have already found the 

 blood-vessels of these animals to be of a delicate structure ; and the 

 veins being wide sinuses whose walls possess little contractility, the 

 fracture of a limb would inevitably produce an abundant and speedily 

 fatal haemorrhage were there not some contrivance to remedy the other- 

 wise unavoidable results of such a catastrophe. Should the claw of a 

 Lobster, for example, be accidentally damaged by accidents to which 

 creatures encased in such brittle armour must be perpetually exposed, 

 the animal at once breaks off the injured member at a particular part, 

 namely, at a point in the second piece from the body ; and by this 

 operation, which seems to produce no pain, the bleeding is effectually 

 stanched*. 



* Mr. Spence Bate gives the following account of this remarkable process: 

 " When a limb is injured, all Crustaceans have the power of rejecting it, except the 

 wound be below the last joint. This is done by a violent muscular contraction, finish- 

 ing with a blow from another limb, or against some foreign body. The amputation 

 is the work of a few seconds, except when they have but recently cast their exuviae ; 

 at such times the wounded limb will sometimes remain for half an hour or longer 

 before it is rejected. 



"The new limb is formed within the shell, where it lies folded up until the next 

 moult, when it appears as a part of the new skeleton, the sac-like membrane which 

 protected it being cast off with the old shell ; and the restored member is larger 

 or smaller, in accordance with the length of time which may exist between the 

 amputation of the limb and the shedding of the skin. The condition in which 

 the limb is at that time remains permanent until the next moult, when the whole 



