151 



available until histochemical methods and electron microscopy made identi- 

 fication of the nervous elements possible. Some works devoted to this prob- 

 lem have recently been published. Ryberg (1973, 1977) and Ryberg and 

 Lundgren (1977) described the cells forming the ectodermal nerve network 

 and the network surrounding the digestive tract as nerve cells of the sea 

 urchin larvae. However, it later became clear that the ultrastructure of these 

 cells nerve is different. The true nervous system of the larva is associated 

 with the ciliated band and muscle elements. The nerve cells are situated along 

 the ciliated band under which pass the axonal tracts — Figure 106 (Burke, 

 1978, 1983a, b). The nervous system of a pluteus develops from the cells of 

 the animal place of the gastrula containing catecholamines (Burke, 1983a, b). 



Figure 106: (a) axonlike projections of cells of the ciliated band of a 60-hr pluteus of 

 Dendraster excentricus. Bar = 0.5 |xm; (b) cells of the ciliated band of a 72-hr pluteus; arrow 

 indicates an axonlike projection from the base of one cell, Bar - - 1 \\m; (c) higher magnification 



of the axonlike process in (b). Bar = 0.5 |im. 

 bl — basal lamina; dcv — dense core vesicles; m — mitochondria; mt — microtubules 



(Burke, 1983). 



Pluteus 11 Stage 



The rate of development up to the pluteus II stage may differ signifi- 

 cantly in various species. Thus Astriclypeus manni (Onoda, 1938) requires 

 only 45 hrs to attain this stage from the moment of fertilization. The devel- 

 opment of stage II in the sand dollars Echinarachnius parma, Scaphechinus 

 mirabilis, and Scaphechinus griseus takes three to five days. Echinarachnius 



