244 



s. GOTO 



matter of fact tlie aboral disc is inclined to both these planes, and 

 consequently that the mnjor axis of the projection ellipse will not be 

 equal to the diameter of the aboral disc, as it is supposed to be in the 

 cut, except in its final stage (c). AVith this correction, however, the 

 above description is accurate. 



Hund in hand with the rotation of the aboral disc, the ond disc 

 (this time an entirely imaginary disc, whose plane passes through the 

 rudiments of the water- vascular system) also rotates in the same direc- 

 tion, but tlie total amount of its rotation is greater thnn that of the 

 aboral disc. This follows from the fact that the oral disc in its initial 

 position makes a negative angle with the sagittal plane, while the 

 aboral disc makes a positive angle with it ; an angle being termed 

 positive when its position is arrived at by turning a plane counter- 

 clockwise, and negative when it is attained by turning a plane clock- 



Cnt 2. 



wise, and 90° beinof considered the limit of rotation in either case. So 

 if we draw projection figures of the oral disc in the same way as we 

 have done with the aboral disc, we shall get more phases than in the 

 case of tlie latter, as shown in cut 2. In this cut the corresponding 

 phases are m:u-ked by the same letters of the alphabet as in cut 1, but 

 with a negative sign when the oral disc in corresponding stages makes- 



