ASTERIAS PALLIDA. 



243 



eccentricities, according as the disc is p:irallel or more or less inclined, 

 to the plane of projection. In the accompanying cut the sngittal plane 

 is represented by the portion above the horizontal line and the trans- 

 verse plane by the portion below it. As stated above, the aboral disc, 

 when it distinctly appears for the first time, makes an angle of about 

 30° with the sngittal plane, and tlierefore we get on the sagittal plane 



an ellipse of com- 

 paratively small, 

 and on the trans- 

 verse plane one 

 p!_ of o-reat, eccen- 

 tricity («) ; when 

 the disc makes 

 an aniile of 45° 

 with both planes 

 we get ellipses 



rram 



Cut L 



of equal eccentricities (6) and when the disc assumes its final 

 position its projection on the sagitttil plane is a simple straight line, 

 while on the transverse phme it comes out ;is a circle. We thus get 

 •different phases, as one may express it, of the aboral disc according 

 to its varying positions with respect to the two principal planes of 

 reference. 



We shall now examine the assumptions, on which the above 

 graphic method has been developed. The first, that the aboral disc 

 is a perfect circle, needs hardly any discussion. We may remark 

 that the ectodermal thickening which gives rise to the aboral side of 

 the star has always a form more or less approaching a flat circular 

 dome. As to the second assumption, that one of the diameters of 

 the aboral disc, imagined as a perfect circle, is parallel to the inter- 

 section of the sngittal and transverse planes, we must say that as a 



