11)0 JOURNAL OF MARINE ZOOLOGY AND MICROSCOPY. 



or the normal limb on the other side. Otherwise all three seem fairly 

 normal in shape. 



I have mentioned that the basal joint is wanting in each of the three 

 abnormal limbs. They are all inserted by means of membrane within 

 one large ring-like coxopod resembling that of the chela of the other side 

 in size, but without the large extension on the outer side that bears the 

 hinge for the rest of the limb. 



An assumption which there is a natural tendency to make regarding 

 this specimen is that the right chela may have been broken off and 

 replaced abnormally by three. The small size of the abnormal limbs 

 somewhat supports this. On the other hand, were the above the case., 

 one would expect to find that the three limbs had not only the eoxopod, 

 but also the basipod in common, and there is, further, no reason why the 

 coxopod should not resemble that of the opposite side, for there is no 

 trace of its having been injured. Probably then, we have to deal with a 

 true variation. Bateson 1 has shown that in such cases as this, of repeti- 

 tion of limbs or parts of limbs in secondary symmetry, one o£ the limbs 

 represents the normal, while the other two are a pair — a right and a 

 left — and further, that the additional pair bear a definite space-relation 

 to one another and to the normal. In this instance it is impossible to 

 determine with certainty which of the three is the normal limb. Still, 

 certain guesses can be made. 



In the first place, FO cannot be the true right chela, since it is a left 

 limb. It lies, then, between FI and O, and of these two O seems the 

 more likely to be the normal, since, as we have seen, FI is closely joined 

 to 0, which is known not to be the normal. For, to quote Bateson, 

 " nearly always the extra legs are more or less compounded together at 

 their point of origin." If, then, be the normal, it has been displaced 

 from its true position by a pair of extra limbs in position VVA 2, If, on 

 the other hand, FI be the normal, the extra pair are in the position DDA. 



A number of cases of paired extra limbs or parts of limbs in secondary 

 symmetry are quoted by Bateson among insects and Crustacea. There 

 is, however, only one case recorded of complete limbs being reproduced 

 in a crustacean, namely, that of the left penultimate walking leg of a 

 Palinuriis vulgaris^. Here the limbs are in a different position from those 

 of our crab ; moreover, the three have a common basipod (with three 

 articular surfaces), while in the crab one is remarkable in having no 

 basipod, and the other two have this joint perfectly distinct in each. 

 Lastly, the distinguishing peculiarity of the present specimen is the 

 small size of the abnormal limbs. 



1. Bateson, he. eit. 



2. See Bateson, he. eit., p. £8l. 



3. Bateson, he. eit., p. 527. Legei", Ann. Sci. ISfat. ZooL, vii. 1. p. ill. pi. 6 (188<i). 



