of the Crinoidea brachiata. 411 



rows of segments, the ambulacral and interambulacral spaces 

 represent the radii and interradii of the Crinoids. In the same 

 way as in the Crinoids, the radii of the Echini do not miite 

 in the dorsal pole, but remain separated therefrom by the 

 vertical apparatus, which, from its position, is the analogue of 

 the base of the Crinoids. The symmetrical Echini acquire 

 their so-called bilateral structure by one of the interradial 

 spaces being distinguished from the rest by the occurrence of 

 the anal aperture ; the radii and interradii, as a consequence 

 of this, arrange themselves in accordance with a radial axis, 

 in the same way as in the Crinoids. In the composition of 

 the vertical apparatus of the Echinus no phenomena occur 

 which are to be compared to the deviations from the regular 

 quinquepartition in the base of the Crinoids j but the vertical 

 apparatus acquires a symmetrical arrangement in another 

 way, by the combination of one of its plates with the madre- 

 poric plate, which is peculiar to the Echini. The ten radially 

 and interradially placed plates of the vertical apparatus may 

 therefore, like the symmetrically divided base of the Crinoids, 

 be arranged in accordance with a dorsal axis cutting through 

 the interradially placed madreporic plate and the opposite 

 radially placed ocellar plate. As in the Crinoids, the dorsal 

 axis does not coincide with the radial ; it has an oblique 

 but regularly fixed position with regard to this. The expres- 

 sion for the position of the dorsal axis in the Echini differs 

 from that in the Crinoids, because, by the presence of the 

 madreporic plate, it acquires a fixed interradial pole, of which 

 the dorsal axis of the Crinoids is destitute. In the symme- 

 trical Echini the law of the position of the dorsal axis is ex- 

 pressed by regarding the madreporic plate as turned towards 

 the right anterior interradius. If we wish to indicate the 

 position in the same way as with the Crinoids, we must place 

 the polar interradius forwards, as in the reversed position of 

 the Crinoidal calyx ; and then we obtain the expression that 

 the dorsal axis of the symmetrical Echini runs from the right 

 adjacent radius to the left abjaeent interradius. This is the 

 position observed in Platycrinus and Taxocrinus among the 

 Crinoids. The regular Echini are comparable to those 

 Crinoids in which a radial axis cannot be distinguished ; for 

 which reason we cannot speak in them of a normal position 

 of the dorsal axis or of the madreporic plate. 



30* 



