166 



siam 



Figure 120: Arrangement of ambulacra! and interambulacral plates in Echinocardium 



cordatum. 



asp — aboral spicule; iam — interambulacral plate; pd — posterodorsal rod; 



pi — posterolateral rod; siam — spicules of the interambulacral plates; spam — spicules 



forming an ambulacral plates;. 



of Salmacis bicolor, mentioned that epaulettes remained for some time after 

 resorption of all the rods and continued to function even though the young 

 urchin had already begun to move using the spines and podia. 



Rudiments of the nervous and reproductive systems appear. Their further 

 development, like that of the ambulacral system, occurs after settling. 



Settling : During development of the juvenile in that period when larvae 

 are still to be found in the plankton, five primary ambulacral podia develop 

 in some urchins, which are usually much larger than the normal ambulacra 



