REPORT OF COMMISSIONERS OF INLAND FISHERIES. 135 



the extra pair of limbs, but also between this pair and the normal 

 appendage. Bateson has formulated these relations under certain 

 principles of secondary symmetry, which he expressed in the follow- 

 ing two rules : 



I. "The long axis of the normal appendage and of the two extra 

 appendages are in one plane; of the two extra appendages one is, 

 therefore, nearer to the axis of the normal appendage, and the other 

 is remoter from it." 



II. "The nearer of the two extra appendages is in structure and 

 position formed as the image of the normal appendage in a plane 

 mirror placed between the normal appendage and the nearer one, at 

 right angles to the plane of the three axes ; and the remoter append- 

 age is the image of the nearer in a plane mirror similiarly placed 

 between the two extra appendages." 



"Transverse sections of the three appendages taken at homo- 

 logous points are thus images of each other in parallel mirrors" 

 (p. 479). 



In the lobster, therefore, according to these rules, in any case of 

 two extra legs or claws, these two extra parts will be a complementary 

 pair, one member of which will belong to the right and the other to 

 the left side of the body. Furthermore, the member of the pair 

 which is nearest the normal limb will always be the one corresponding 

 to the other side of the body. 



Let us now see how the specimens with extra processes described 

 in the present paper will conform to these principles. With regard 

 to the universal character of these rules, Bateson observes that "It 

 would not be true to assert that these rules are followed with mathe- 

 matical precision, but in the main they hold good" (p. 482). And 

 it is remarkable to what extent these rules hold for the present speci- 

 mens; for it will be seen that the characteristic relations which might 

 have been predicted on the basis of Bateson's principles are actually 

 found in almost every case. 



For our present purpose these specimens arrange themselves con- 

 veniently under two groups : 



