480 



MERISTIC VARIATION. 



[part I. 



set to "Ventral" the two supernumeraries will turn their dorsal surfaces to each 

 other, and so on. The model SL thus rotates twice on its own axis for each 



Fig. 153. A mechanical device for shewing the relations that extra legs in 

 Secondary Symmetry bear to each other and to the normal leg from which they 

 arise. The model R represents a normal right leg. SL and SR represent respect- 

 ively the extra right and extra left legs of the supernumerary pair. A and P, the 

 anterior and posterior spurs of the tibia. In each leg the morphologically anterior 

 surface is shaded, the posterior being white. R is seen from the ventral aspect and 

 SL and SR are in Position VP. 



revolution round R, but the surfaces of the model SR always remain parallel to those 

 of the model R. In every possible position therefore each model is the image of its 

 neighbour in a mirror tangential to the circle of revolution. In the figure the models 

 have the position they should have if arising postero-ventrally. Here the plantar 

 surface of SL is at right angles to the plantar surfaces of the other two legs. 



►Since at each radius the relative position of the legs differs, it is 

 possible to define these positions by naming the radius. This will be 

 done as shewn in Fig. 154. In this diagram imaginary sections of the 

 legs are shewn in the various positions they would assume at various 

 radii. The central thick outline shews a section of the normal leg, a 

 longer process distinguishing the anterior surface from the posterior. 

 The radii are drawn to various points D, A, V, P, representing 

 the dorsal, anterior, ventral and posterior positions respectively. In- 

 termediate positions may be marked by combinations, DA, VVP, 

 &c, using the system employed in boxing the Compass. 



On several of the radii ideal sections of the extra legs are shewn in 

 thin lines, the shaded one being the nearer and the plain one the 

 remoter. M 1 and M 2 shew the planes of the imaginary mirrors. 



The manner in which the pair of extra limbs are compounded with 

 each other in their proximal parts, and with the normal limb at their 



