346 Sir John Graham Dalyell, Bart., on 



if it be a defective specimen, mutilated perhaps of both large 

 claws, or of the eight limbs, all these ten organs will come 

 in perfect and entire on the first subsequent exuviation, as 

 an integral portion of the new shell. On the other hand, no 

 subsisting shell which is mutilated seems ever to acquire a 

 new limb. There is naturally a great disparity in the size 

 of the claws of various genera. It is difficult to explain 

 either the formation and position of the new shell within the 

 existing animal, or how it escapes on exuviation. 



For a long time I concluded that the new parts were de- 

 rived from the old, that a claw was generated within a claw, 

 a limb within a limb, the eyes within the eyes ; thence con- 

 curring in the prevalent opinion, that on exuviation each was 

 withdrawn from the pre-existing organ as from a sheath. 

 Nature seems to conduct her operations otherwise. But the 

 means are most obscure. 



The adult of the common crab, Cancer Pagurus, is of a 

 reddish brown colour, darker or lighter, the claws always 

 tipped black ; but some of the young are naturally of the 

 purest white, which remains long unsullied. This is not in- 

 cident to confinement, which has no effect on colour. 



A young white specimen, C, was taken among others on 

 September 29. The body might have been circumscribed by 

 a circle, nine lines or three quarters of an inch in diameter, 

 and the extended limbs by one and a half inch in diameter. 

 Its first exuviation ensued on November the 8th ; the second 

 on the 30th of April following. The shell now produced 

 subsisted until September 12, when another exuviation took 

 place, introducing a new shell of such pure and transparent 

 white that the interior almost shone through it. All the shells 

 were white, and somewhat larger successively. This last 

 shell of September 12 subsisted until March 29, being 197 days, 

 when it was evacuated by another exuviation, introducing 

 its contents, D, to view. The new animal, as I must call it, 

 D, had only the two claws ; all its eight limbs were deficient. 

 Resting on the breast, I did not at first discover the fact, but 

 the creature presented a very strange and uncouth aspect. 

 However, it fed readily and proved very tame, though help- 

 less, often falling on its back, not able to recover itself from 



