GROWTH OF NEW PLATES. 



In the Spatangoids and Clypeastroids the pores are never arranged in arcs ; 

 the ambulacral plates are limited to their simplest expression, — a series of 

 single plates arranged symmetrically on the two sides of a median axis ; 

 each plate is perforated, either with one or two pores, to carry only a 

 single tentacle. We find in these suborders no difficulty in understanding 

 the mode of increase of the ambulacral areas. Additional plates are con- 

 stantly added at the apical pole, which push down, towards the actinostome, 

 the older, previously formed plates (PL XI, PL XII.). The growth of 

 the upper part of the ambulacral system above takes place independently of 

 the increase in number of the plates of the interambulacral areas. At the 

 actinal end of the ambulacral system the same phenomenon of independent 

 growth takes place by the crowding together of the ambulacral plates to 

 form phyllodes. The intermediate parts of the ambulacral system between 

 the phyllodes and the petals retain their relations to the adjoining inter- 

 ambulacral plates during their growth, as the number of new ambulacral 

 plates formed is small compared to that added at the apical system to form 

 the petals. So that in the Clypeastroids, as well as in the Petalosticha, the 

 number of plates in the two areas increases independently, but only at the 

 abactinal system ; the plates increasing in size round the edges as fast as re- 

 quired by the increase in diameter of the test. Plates of both systems are 

 constantly passing from the region above the ambitus to the actinal surface, 

 the periphery not being a constant one even in the flat Scutellidae. 



In the Echinonidae the same simple mode of independent increase of the 

 plates of the two systems is observed. In the Desmosticha, the Ciclaridae 

 alone have the same simple mode of growth, and, owing to the small number 

 of plates of the interambulacral area, the growth of the ambulacral system 

 is plainly shown to be entirely disconnected from that of the interambulacral 

 system ; new plates are constantly formed at the abactinal extremity of the 

 two areas immediately adjoining the genital plates (PL VI. f. 2l). In many 

 genera, even in the adult, the abactinal part of the ambulacral system 



