178 



Pluteus II stage 



From the time of appearance of the third pair of arms, the posterodorsal, 

 the larva enters the pluteus II stage. A calcification center arises on each side 

 at the base of the postoral arms and forms a single triradiate rod that later 

 transforms into a short rod. As it grows, a projection develops alongside the 

 postoral arm on the dorsal side of the larva. This projection is the posterodorsal 

 arm. Simultaneous with the development of the new pair of arms, the irmer 

 transverse rods begin to interconnect. The secondary basal rods also begin to 

 enlarge but do not attain the full length of the basal rods; approximately 

 midlength of the latter they bend inward and cross. A short dorsal arch 

 arises on the dorsal side above the stomach. The basal part of the body 

 gradually enlarges and the spines at the distal ends of the basal rods break 

 off. This process becomes perceptible when the third pair of arms reaches the 

 size of the two earlier pairs and the larva is as long as 500-700 |im (Figure 

 130). 



Figure 129: Strongylocentrotus nudus. 



Pluteus I stage. 



Legend same as in Figure 103. 



Figure 130: Strongylocentrotus nudus. 

 Pluteus 11 stage, 

 da — dorsal arch. 

 Remaining legend same as in Figure 107. 



Pluteus III stage 



The ends of the basal arch elongate into a pair of new rods, which support 

 the processes appearing above the oral opening. These rods are termed the 

 preoral. With their appearance, formation of the larval skeleton terminates 

 and the larva enters the pluteus 111 stage. In the dorsal and ventral views, at 

 the level of the esophagus, dense ciliated bands of the ectoderm gradually 



