161 



Figure 1 14: Aboral view of the young urchin Strongylocentrotus intermedins. 

 gp — genital plate; m — madreporic plate; op — ocular plate; pr — periproct. 



The skeleton of the oral apparatus (Aristotle's lantern) begins to form 

 very early. Dental cones appear after the emergence of the first ambulacral 

 plates. Rudiments of the alveoli and epiphyses appear before settling (Figure 

 119). 



Formation of test in the heart urchins (subclass Irregularia, order 

 Spatangoida): According to Gordon (1926a) and Kryuchkova (1979a), in 

 Echinocardium cordatum during the development of the definitive test, 

 interambulacral plates and their spines are the first to appear (Fig. 120). 



The ocular plates are not associated with the larval skeleton. The third, 

 first and fifth ocular plates appear before settling while the second and fourth 

 plates appear after the juvenile individual settles. 



Development of the genital plates is associated with the remains of the 

 larval skeleton. The second plate, as in all urchins, develops around the 

 process of the dorsal arch. The third, fifth and fourth plates appear around 

 fragments of the left posterodorsal, left postoral, and aboral rods respectively. 

 Only the first genital plate develops independent of the larval skeleton (Figure 

 121). 



