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Formation of test in regular sea urchins (subclass Regularia): In 

 Strongylocentrotus nudus and Strongylocentrotus intermedius the interambulacral 

 plates and their spines appear first. Following the development of the first 

 interambulacral plates, the twin rudiments of the first ambulacral plates appear 

 (Figure 113). Then the ocular plates develop: four of them independent of the 

 larval skeleton, with the first one around the fragment of the left posterodorsal 

 rod. Development of the genital plates is associated more closely with the larval 

 skeleton. As in Arbacia pustulosa (= A. lixula), Strongylocentrotus lividus 

 (Ubisch, 1913), and Heliocidaris crassispina (Onoda, 1931), in Strongylo- 

 centrotus nudus genital plates 1, 4 and 5 are also modified from the plate 

 constituting the base of the pedicellariae, which remain on the juvenile indi- 

 vidual for some time after it settles. In Strongylocentrotus intermedius the 

 plates that developed in the larvae III stage around the left posterodorsal and 



Figure 113: Arrangement of the ambulacral and interambulacral plates in Strongylocentrotus 



intermedius. 

 am — ambulacral plate; iam — interambulacral plate; op — ocular plate. 



