1890. J REPETITION Ob l*.\KTS IN ANIMALS. .")81 



plants, are rare amongst animals. This variation is especially 

 interesting from the fact that a precisely similar case of the trans- 

 formation of the third maxillipede (left) into a chela has been 

 already observed in C. yagurus (Cornish, T., Zoologist (b), viii. 

 p. 349). 



II. — Cases of Repetition of the Pincers of the Chelce in Crabs 

 (Cancer pagurus). 



A & K. These two specimens were brought by fishermen to the 

 Plymouth Laboratory. The greatest measurement of the carapace 

 was in each case about five inches. The one specimen was a male, 

 but the sex of the other was not noted. With the exception of the 

 varying structures about to be described, the animals seemed normal 

 and healthy. In A the chela of the right side had the form shown 

 in fig. 2, A (p. 582), which represents the limb seen from the out- 

 side. The dactylopodite bears two supernumerary, fixed processes. 

 Whether the outer pair of processes which curve towards eaOh other 

 are the extra ones, or whether two processes have grown up on the 

 inside of the dactylopodite, cannot be affirmed ; but the lattet 

 seems more likely. If this is the true interpretation, it will be 

 seen that one of the extra processes curves towards the " index " of 

 the limb, while the other turns to meet the dact3rIopodite. 



Though the fact may have no relation to the presence of this 

 supernumerary structure on the right side, it should nevertheless 

 be mentioned that the chela of the left side, which was otherwise 

 perfect, had lost its dactylopodite. The socket in which the dactylo- 

 podite usually moVes was filled with a plate of hard shell, but 

 whether the joint had been lost by injury or had been congenitallv 

 absent could not be affirmed. Since mutilated limbs are generally 

 thrown off by Crabs, the presence of such a chela without the 

 dactylopodite is so far evidence that this joint had not been lost by 

 an accident. As, however, according to the observations of Heineken 

 (Zool. Jouru. vol. iv.), such mutilated parts are occasionally retained, 

 much stress cannot he laid on this consideration. 



The left chela of B is shown in the figure as seen from the inside. 

 The dactylopodite bears a thick process which divides peripherally 

 into two stumpy projections which bear teeth on their inner faces-. 

 These projections are like the normal pincers in consistency and 

 Colour. 



C; This specimen was kindly leht to me for description by 

 Mr. J. Carter, F.E.C.S., of Cambridge. It is the right chela of a 

 Cancer pagurus. In it the repetition of parts is far more extensive 

 than in either of the preceding specimens. As is shown in fig. 2, C, 

 it bears two dactylopodites, each complete in all respects, and to 

 each of these dactylopodites is opposed a fixed process. In addition 

 to this, one of the two dactylopodites is partially divided longitudinally 

 into two, and at its free end terminates in a pair of toothed pro- 

 cesses. The teeth on these processes are continued downwslrds on 

 the inner surface of the joint in two complete rows. The total 

 number of points borne by this claw is five. 



Proc. Zool. Soc— 1890, No. XXXiX. 39 



