99 



ON A CANCER PAGURUS WITH SUPERNUMERARY 



CHELJE. 



BY L. A. BOBBADAILE, MA. 



{Lecturer in Natural Sciences at Selwyn College, Cambridge) . 



By the courtesy of Mr. J. Horn ell, Director of the Jersey Marine 

 Biological Station, I am permitted to describe a remarkable specimen in 

 the possession of that institution. This is a female of the edible crab, 

 taken in the Spring of 1897, at low water on the shore in Jersey. The 

 carapace measures about 8 cm. by 12 5 cm. The peculiarity consists in 

 the fact that the great chela on the right side is wanting, and is replaced 

 by three small limbs resembling the normal cliela, save in the minor 

 points to be presently described. The other limbs are normal, though 

 the second walking leg on the right side, and the moveable " finger " of 

 the left chela are broken off. Of the three abnormal appendages two 

 are directed forwards and the third outwards. Again, of the two 

 forwardly-directed, one may be distinguished as the outer and the other 

 as the inner. I shall refer to these by the letters FO and FI, while the 

 outward limb will be called 0. FO faces outwards, FI faces inwards, 

 O faces forwards. Thus the three have the positions described by 

 Bateson 1 as found in such cases of supernumerary limbs, or parts of 

 limbs, in secondary symmetry —which is that each of two adjacent limbs 

 bears to the other the relation of an object to its image in a mirror. 

 FO and FI are each complete save for the basal joint or " coxopod." 

 wants also the basipod, the true second joint, fused, in this limb, to 

 the next or "ischiopod," the true third joint. When a limb is cast off 

 under the influence of fear, the break always takes place in the middle of 

 the seeming second joint, really at the line of fusion of the basipod and 

 ischiopod. "We shall see that this fact has an important bearing on the 

 case before us. 



FO and FI lie close together back to back, but are quite free from 

 one another, save in the basipods. This joint is distinct in each limb, 

 but is connected with the similar joint in the fellow limb by a bridge of 

 calcified material, similar to that composing the joint, at its far end. 

 Centrally of this bridge the two basipods are connected by membranous 

 skin. The basipods are not exactly similar in their ridges, etc., nor does 

 either exactly resemble that of the normal chela on the opposite side. Of 

 the other joints of the three limbs, the fourth joint (meropod) of FI has 

 a somewhat shrunken and distorted appearance, while the moveable 

 finger is bent dorsally. The sixth joint or hand of FO and is some- 

 what rougher and has its ridges more prominent than that of either FI 



1. W. Bateson, " Materials for the Study of Variation," p. 479. London, 1894. 



