134 REPORT OF COMMISSIONERS OF INLAND FISHERIES. 



with abnormal extra processes. The first of these categories would 

 include all those cases in which two limbs or parts of limbs are iden- 

 tical copies of the normal appendage for which they stand, and con- 

 sequently belong to the same side of the body, or, in Bateson's termi- 

 nology, are in "succession." The second category would include 

 all cases in which two limbs or parts of limbs are mirror images of 

 each other; i. e., they represent a complementary pair of right and 

 left appendages. 



In regard to the first category, no authentic case is known which 

 conforms to it. In every case so far recorded "no right arm is 

 ever succeeded on the same side of the body by another arm properly 

 formed as right and no crustacean has two right legs in succession, 

 where one should be" (Bateson, 529). Bateson records nine cases 

 of crustacean limbs apparently double in the sense of the second 

 category; and Zeleny ('05) has carefully described the "regeneration 

 of a double chela in the fiddler crab." But Bateson hesitates to 

 admit the existence of such double structures, and that there should 

 be such a thing as a double limb (in the sense of this second category) 

 "has always," he remarks, "seemed to me most strange;" he thinks 

 it possible rather that one of the members of such a pair may generally 

 be regarded as itself really double. Bateson, therefore, was able to 

 exclude nearly all cases of so-called double limbs from these two 

 categories, and established the important point that the great 

 majority of abnormal crustacean limbs — including most cases of 

 apparent double appendages — can be brought under a third category 

 of variations; a category, namely, in which the extra limb or extra 

 parts of a limb are themselves morphologically double. 



In the course of the following discussion it will become evident that 

 all of the cases of extra processes described in this paper may be 

 brought under this third category. 



A closer study of this class of abnormal structures brings us to 

 a discussion of certain characteristic principles. It has been dis- 

 covered that there is a remarkable degree of regularity in the morpho- 

 logical and spatial relations, not only between the two members of 



