Abnormal Appendages in Crahs, 401 



from near the base to about the middle of its length. On 

 the ventral side the dark colour of the distal portion of the 

 dactylus shades off gradually near the base as in the normal 

 cheliped, but on the dorsal side it ends more or less abruptly 

 along an irregular line slightly raised above the surface, much 

 as it does in the walking-legs. The low rounded tubercles 

 on the opposed edges of the fingers agree in form, and appa- 

 rently also in number, with those of tlie normal cheliped of 

 the same side. 



The chief points of interest in connection with this speci- 

 men appear to be two. While it is a clear case of homceosis, 

 to use Bateson's term, it is an imperfect one. The modified 

 limb, in assuming the characters of a cheliped, has not parted 

 altogether with those proper to a walking-leg. The brushes 

 of hair on the dorsal or outer edge of propodus and dactylus 

 are closely similar to, although less developed than, those of 

 the other walking-legs, and they are quite unrepresented on 

 the cheliped. The sharp restriction of the coloured area 

 of the dactylus on the dorsal side is also a character of the 

 walking-leg. It is to be noted that both these characters 

 are on the dorsal or extensor aspect of the limb, the only 

 feature pointing in the same direction which can be 

 discovered on the ventral or flexor aspect being the scattered 

 hairs on the margin of the propodus. Briot (C. R. Soc. 

 Biol. Paris, Ixiv. 1908, p. 1182), describing a case of homoe- 

 otic resemblance to a thoracic leg in the first abdominal 

 appendage of a male crayfish, has called attention to a similar 

 coexistence of incongruous characters on the outer and inner 

 sides of the appendage. 



The second point which seems worthy of notice in this 

 specimen is that the abnormal limb is not adjacent to the 

 limb which it ''mimics,'-' the first walking-leg, which is 

 normal in every respect, coming between them. The only 

 analogous case recorded among Crustacea is that of the 

 Asellus described by Bateson (Proc. Zool. Soc. London, 

 1900, p. 268), which had an antennule replaced by a man- 

 dible. Bateson notes that the cases most nearly approaching 

 this are those of insects which have the antenna replaced by 

 a foot. The very remarkable case of a shore-crab having a 

 walking-leg on the abdomen, described by Bethe (Journ. 

 Mar. Biol. Assoc, iv. p. 144, 1896, and Arch. Entwmech, iii. 

 p. 301, 1896), appears to belong to a different category 

 altogether. 



It is an obvious suggestion that reversion or atavism may 

 be concerned in this abnormality, since the limb in question 



