DESCRIPTION OF AN ABNORMAL LOBSTER CHELIPED. 26 1 



in the case of the claw, the torsion of the more proximal parts 

 of the limb appears also to have been hindered by the abnormality. 



Secondary Symmetry: Dextro-sinestral Relations. — Like most ab- 

 normal crustacean appendages with extra processes, the present 

 case falls into the category "in which the extra limb or extra 

 parts of a limb are themselves morphologically double." Fur- 

 thermore, in accordance with the rules of secondary symmetry 

 laid down by Bateson (1894, p. 479), the normal appendage and 

 the extra parts lie in the same plane^ and "the nearer of the two 

 extra appendages is in structure and position formed as the image 

 of the normal appendage in a plane mirror [M, Fig. 6] placed 

 between the normal appendage and the nearer one, at right 

 angles to the plane of the three axes ; and the remoter appendage 

 is the image of the nearer in a plane mirror [M', Fig. 6] similarly 

 placed between the two extra appendages." Thus the two extra 

 claws are morphologically a pair and belong to opposite sides 

 of the body, the inner or proximal one, according to the above 

 rule, being a morphological right (the primary claw being a left), 

 and the outer one a left. This is evident in the diagram (Fig. 6), 

 where it will be noticed by the shading that the relations of the 

 posterior (shaded) and anterior (not shaded) halves are reversed 

 in the middle claw. In one respect, however, the middle claw 

 is not a right ; for the normal right claw of this lobster was in all 

 probability of the "nipper" type, the left being a "crusher." 

 As a matter of fact the teeth on both the indices and dactyls of 

 all three of the claws of the abnormal appendage (IL, DL; 

 I'R, D'R; I'L, D'L) are of the "crusher" type. Thus it is evi- 

 dent that while the rules of secondary symmetry hold good for 

 the spatial relations they do not apply to the character of the 

 parts of the appendage when these normally differ on the two 

 sides of the animal.^ 



Bateson (1894, p. 479, et seq.) has shown that a definite rela- 

 tion obtains between the position which the extra parts assume 

 to each other and to the primary appendage, according to the 

 side of the appendage from which they arise. The theoretical 

 positions he has illustrated plainly by means of a diagram (loc. 



^ This plane (Fig. 6, pi'.) is bent in the present case, owing to torsion. See later. 

 ^ Emmel (1907, p. no) found the same thing to be true in his "specimen No. 5." 



