ECHINODERMA GENERAL DESCRIPTION 



appears to have been the passage of the anus to the aboral pole, 

 the madreporite remaining eccentric and marking interradius CD ; 

 whether this CD is identical with CD of Echinocystis, is another 

 question ; then the anus moved away from the pole in the direc- 

 tion of radius B, so that the anal plane made an angle of 72 with 

 the M plane. This new plane (interradius DE, radius B} is termed 

 by Cuenot the "Echinid plane"; Loven (1884) has shown that 

 the plates of the five interradii in Echinoidea are disposed sym- 

 metrically with reference to this plane. The sagittal plane of many 



FIG. XVIII. 



Planes of symmetry in Echinoderma (partly after Cuenot). 1, Crinoid ; 2, Asteroid ; 

 3, Holothurian ; 4, Regular Echinoid ; 0, Irregular Echinoid. As, anus ; M, plane passing 

 through madreporite ; Ech, Echinid plane ; L, Loven's plane. For other letters, see adjoining 

 text. 



other sea-urchins, notably the heart-urchins and their allies (Fig. 

 XVIII. 5), i.e. the plane passing through mouth, anus, and apical 

 pole, corresponds with neither the M plane nor the Echinid plane, 

 but passes through radius D and interradius AB ; Cuenot calls it 

 " Loven's plane." The bivium (AB) and trivium (C, D, E) of a 

 heart-urchin are therefore in no way identical with those parts in 

 a Holothurian, a Crinoid, or a Stelleroid. In those starfish that 

 have an anus (Fig. XVIII. 2), that organ, as shown by Ludwig, is 

 in interradius BC ; this with the vertical axis marks the " Asterid 



