HIGHWAY OF ANIMAL EVOLUTION 243 



FIGURE 12. DIAGRAMS OF AN ARACHNID-LIKE ANIMAL SEEN FROM THE 

 HAEMAL SIDE, to illustrate the mechanism of reorientation in the triaxial 

 system. In A, the balanced effects, right and left, of a luminous point at a, 

 are indicated, and the unbalanced effects of a luminous point at a\ In B, a 

 balanced condition is automatically reestablished in relation to the point a, 

 by the more vigorous action it has produced in the left appendicular muscles 

 of the head region, and in the myotomes on the right side of the trunk 

 and tail regions. 



Note the difference in the action of the head and trunk muscles neces- 

 sary to bring about this bodily reorientation to an asymmetrically placed 

 stimulus. 



the radial lines of gravitational action, as it normally 

 all the paired receptors and affectors are then, and 



is 



only then, rightly oriented to the great horizontal plane 



