181 



Pluteus II stage 



Similar to S. nudus, the third posterodorsal pair of arms develop during 

 stage II in this species also. The dorsal arch becomes discernible and is much 

 narrower than in S. nudus. By the end of stage II, the clavate tips of the basal 

 rods break off but the position of the rods remains the same; hence the hind 

 end of the larva appears almost unaltered (Figure 133). On the 8th-9th day 

 of development, the ends of the basal arch elongate, giving rise to the preoral 

 rods. The pluteus II stage then becomes the pluteus stage III. 



Pluteus III stage 



The base of the larval body gradually enlarges and parts of the basal rods 

 contained therein gradually reduce. Ciliated epaulettes appear, which are 

 denser than in S. nudus and lie closer. The larva looks as if it has two 

 powerful ciliated bands. Pedicellariae do not form in S. intermedius. The 

 larval body is high set, resembling a truncated cone from which four pairs of 

 arms arise. All the skeletal rods are simple and armed with numerous short 

 spines. The larva soon loses its transparency, becoming bright red. This 

 change of color is due to the disappearance of the earlier scattered crimson 

 pigment granules, which are replaced by more numerous red granules. Now 

 the tips of the arms and the larval body are so filled with red granules that 

 the skeletal rods are no longer discernible. However, at higher microscopic 

 magnification, the reticular structures can be seen underneath the layer of 

 pigment on both sides of the stomach and at the base of the larval body. On 

 removal of the soft tissue, these structures are found to consist of lattice 

 plates, which have formed around the dorsal arch and left and right posterodorsal 

 and postoral rods. Another small plate is situated at the base of the larval 

 body. The fate of these plates was discussed above in the section on meta- 

 morphosis and formation of the definitive test. At the time of plate formation, 

 the length of the larva is 750-900 |im (Figure 134). 



Ecology 



In Peter the Great Bay, larvae of the pluteus I stage begin to appear in 

 mid- July, when the water temperature in the surface layer is 16.5°C, and 

 continue to be found until early September with some interruptions. The first 

 juvenile individuals are found in early August. Large-scale settling is ob- 

 served in mid- August. In southern Hokkaido, according to Fuji (1960b), spawn- 

 ing of S. intermedius is observed from mid- August to late December. Accord- 

 ing to Kawamura and Taki (1965), who investigated the population of these 

 urchins near Rebun Island, spawning in this region occurs from early August 

 to mid-September. 



