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A Carcinus with a right-handed walking -leg on the 



left side of the abdomen. 



PRELIMINARY NOTE. 



By 

 Albrecht Bethe. 



Amongst a great number of crabs collected in Plymouth Sound for 

 my studies of the central nervous system, one specimen was found 

 with a very interesting abnormality. The thorax of the crab — a 

 female — is normal. The length of the carapace is 47 mm., the breadth 

 64 mm. The claws and legs are in the right positions, and of normal 

 proportions, with the exception of the fourth leg on the left side, which 

 is smaller than usual. In my opinion, this leg was broken some time 

 before the crab came in, and was not yet quite regenerated. The first 

 four segments of the abdomen are also normal. Each has a couple 

 of ;pedes spurii. To the fifth segment, which in the crab is always 

 legless, a large leg is fastened on the left side. (Fig. 1.) 



This leg is, as one can see at a glance, a real walking-leg, and, what 

 may at first sight seem strange, not a left-handed, but a right-handed 

 leg. Comparing it with the other walking-legs, one can see that this 

 additional leg corresponds to the second and third of the thorax. Not 

 only are the positions of the hairs and little pits the same as in those 

 legs, but also the proportions between the single joints and the angles 

 which form the axes of the joints. 



Owing to the pressure of the large leg, the left side of the fifth 

 abdominal segment is a little bent on both sides, and the exopodite 

 of the left pes spurius of the fourth segment is stunted, so that it is 

 not half as long as the exopodite of the other side. When alive, 

 the leg was motionless, but it was sensitive. 



When first I saw the crab, I imagined this surplus leg must be 

 innervated by a nerve coming from a surplus half-ganglion of the 

 right side. And this indeed proved to be the case. A dissection made 



