158 



Figure 112: Psammechimis miliaris (Ryberg and Lundgren. 1979). 

 Arrows show disposition of pigment cells in the pluteus. 



Thickening of the floor of the amniotic sac constitutes the imaginal disk of 

 organs of the oral half of the definitive urchin. As in sea stars, the oral-aboral 

 axis of the definitive animal lies at an angle to the principal axis of the larva. 

 Metamorphosis in sea urchins is markedly catastrophic in nature. Cidaroid sea 

 urchins might be an exception to this rule (Emlet, 1988). 



Skeleton : Development of the definitive test in the amniotic sac is ac- 

 companied by resorption of rods of the larval skeleton beginning, first of all, 

 with rods supporting the larval body. The resorption of rods supporting the 

 arms is observed only after the juvenile individual has settled on the substrate 

 and moves rapidly for a few hours (Fewkes, 1893; McBride, 1903, 1914, 

 1918; Ubisch, 1913; Onoda, 1931; Aiyar, 1936). 



During the development of the definitive test the coronal elements de- 

 velop first, i.e., the interambulacral and ambulacral plates and spines. Forma- 

 tion of the oral and aboral sides terminates only after the juvenile individuals 

 have settled on the substrate (Gordon, 1926a, b, 1928; Onoda, 1931; 

 Kryuchkova, 1979a, b, c). 



