242 s_ GOTO; 



irregularly curved, but if, for simplicity's sake, we regard it as a plane 

 disc, it makes, when first formed, an angle of about 30° with the 

 sagittal plane of the larva. As metamorphosis goes on, however, it 

 gradually rotates counter-clockwise ^\ so as to make a g-reater ano-le 

 with the same plane. This rotation continues until the aboral disc 

 comes finall}^ to be at a right angle to the sagittnl plane. These 

 statements are abundantly proved by Figs. 13-14, PI. XIX and 

 Figs. 17 & 18, Pi. XX, as well as by many sections which are not 

 reproduced here. 



The primary water-pore when first formed is on the left side, but 

 later it is found in the median line or even slightly but distinctly on 

 the right side (Fig. 19, PI. XXI) — a position which it keeps perma- 

 nently. Xow, from the above description it will be clear that the 

 sagittal plane of the larva cuts the disc of the star at riglit angles and 

 passes through the water-pore and the centre of the disc, that is to say, 

 the sagittal plane of the larva and the plane of bilateral symmetry of 

 the star are coincident. The arms of the starfish may therefore be 

 justly spoken of as the median ventral, the right and left dorsal, and 

 the right and left ventral, arms. It need hardly be added that the 

 oral side is anterior, and the aboral side posterior. 



The rotation of the aboral disc above described will perhaps be 

 more clearly understood and can be graphically represented by making 

 two arbitrary suppositions, which although not accurately realised in 

 nature, still are, in my opinion, perfectly allowable in the interest of clear 

 exposition, viz., that the aboral disc is a perfect circle and that one of 

 its diameters is parallel to both the sagittal and transverse planes. If 

 then we project this circular disc by the usual method on the sagittal 

 and transverse planes we shall get a circle or ellipses of difi*erent 



1). In describing the rotation of planes we shall suppose, vinless othei-wise stated, that the 

 observer is looking from the ventral side of the larva. 



