ASTERIAS PALLIDA. 245 



a negative angle with the sagittal plane, and a being evidently the 

 phase when the oral disc is parallel to the sagitt;ü plane. Phase a is 

 simply omitted. 



The correction needed for the aboral disc is entirely superfluous 

 in the case of the oral disc, as one of its diameters is always practically 

 parallel with the intersection of the sagittal and transverse planes. 



The rotation of the disc above described has been observed in a 

 bipinnaria Iw Metschnickotf ['69, p. 3(3 et fort'l; in Asterina (jihhosa by 

 MacBride ['96, p. 355], who enumerates it as one of the characteristic 

 features of its development ; and is also well shown in one of Ludwig's 

 figures ['82, PI. YI, Fig. 80 = MacP>ride '96, PI. XVIII, Fig. 14]; but 

 its significance has evidently escaped the attention of the latter, and 

 MacBride has draw^n, it seems to me, an entirely mistaken inference. 

 He says, "Tlie adult plane makes an angle of about 70° or more with the 

 larval plane ; but without any very serious error, it may be regarded, 

 for purposes of description, as at right angles to it : thus the direction 

 right to left, according to the larval plane, becomes al3oral to oral ac- 

 cording to the adult plane, and dorsal to ventral according to the 

 larval plane is nearly parallel to the adult plane " ['96, p. 357]. J]y 

 larval plane MacBride means the frontal plane of the larva. But the 

 adult plane makes a right angle not only with the frontal, but 

 also with the sagittal, plane of the larva ; therefore the direction right 

 to left as ivell as dorsal to ventral in the larva is parallel to the adult 

 plane. Bury ['95, p. 70] also assumes that in Bipinnaria asterigem 

 the sagittal plane of the larva and the plane of the star coincide ; but 

 in view of the facts brought out in the above discussion, I think I 

 may say that this will also be shown, on further study, not to be the 

 case, but that the two planes make a right angle with each other. 

 Again, if the orientation of the body above explained is correct, then 

 the aboral pole of the asterid and that of the crinoid are, as justly 



