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Show 1390.J REPETITION OF PARTS IN ANIMALS. 581 plants, are rare amongst animals. This variation is especially interesting from the fact that a precisely similar case of the transformation of the third maxillipede (left) into a chela has been already observed in C. pagurus (Cornish, T., Zoologist (3), viii. p. 349). • II.-Cases of Repetition of the Pincers of the Chelce in Crabs (Cancer pagurus). A & B. 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 malo, 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 shdwil in fig. 2, A (p. 582), which represents the limb seen from the outside. The dactylopodite bears two supernumerary, fixed processes. Whether the outer pair of processes which curve towards each other are the extra ones, or whether two processes have grown up on the inside of the dactylopodite- cannot be affirmed; but the latter 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 dactylopodite. 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 leftside, which was otherwise perfect, had lost its dactylopodite. The socket in Which the dactylopodite usually moves was filled with a plate of hard shell, but whether the joint had been lost by injury or had been congenitally 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. Journ. vol. iv.), such mutilated parts are occasionally retained, much stress cannot be 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.R.C.S., of Cambridge. It is the right chela of a Cancer pagurus. In it the repetition of parts is far more extensive thah 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 processes. The teeth On these processes are continued downwards on the inner surface of the joint in two complete rows. The total number of points borne by this claw is five. P R O C ZOOL. Soc-1890, No. XXXIX. 39 |
