THE ECHINODERMATA 263 



plate, and each plate extends across half the width of the 

 amb. Such plates are called simple primaries. Lower 

 down compound plates appear, each with several pore- 

 pairs. The development of living forms shows that such 

 plates arise by the fusion of simple plates, and sometimes 

 the fused sutures can be traced in a fossil. When this 

 can be done in Hemicidaris, it is found that three plates 

 fuse together : the middle one carries most of the large 

 tubercle and may be the only one that occupies the full 

 width of its column (primary plate), the other two only 

 covering about two-thirds of that width (demi -plates}. 



Growth of the corona takes place (i) by intercalation 

 of new plates at the aboral end of the ambs and inter- 

 ambs, and (2) by increase of size (if necessary, with 

 alteration of shape) of existing plates. The early-formed 

 coronal plates thus get thrust farther from the apical 

 system as growth proceeds. Sometimes plates next the 

 peristome may undergo resorption. Thus, though in- 

 dividual plates grow by accretion, the corona as a whole 

 does not, but grows by a more complex process of intus- 

 susception. Consequently the way in which the adult 

 corona illustrates its own development is the reverse of 

 that seen in molluscs and brachiopods. In echinoids 

 the earliest-formed coronal plates, instead of illustrating 

 the earliest stage of growth, are the most highly de- 

 veloped ; while those which have been latest formed show 

 the simplest characters. Thus it is that in Hemicidaris 

 the ambulacral plates next to the apical disc are simple, 

 while those near the peristome are compound. 



The ambs and intera.mbs end orally around a large 



