734 



ON THE YOUNG STAGES OF ECHINI. 



ambulacral plate; the same is the case with other Oligoporidae, as limited 

 by Desor, showing that the division he has made, although convenient as 

 a key for the easier grouping of genera, is yet not strictly reliable, the mode 

 of growth of many Polyporidae showing in their young stages that they 

 have but a small number of pores (Hipponoe, Mespilia) for each ambulacral 

 plate, which places them among the Oligoporidae; but, owing to the inde- 

 pendent growth of the plates of the poriferous zone, they seem in older 

 stages to belong to the Polyporidae. In the chapter on the water system I 

 have given an account of the mode of increase of the number of plates 

 of the poriferous zone in several genera. In all of them the independent 

 development of the plates of the ambulacral anil interambulacral systems is 

 very marked ; their independent growth has, subsequently to the publica- 

 tion of my Preliminary Report, been observed by ho ven. 



Fig. 68. 



