186 



Figure 138: Scaphechinus mirabilis. Pluteus 1 stage. 



A — structure of the larval skeleton; B — connection of the basal and secondary basal rods. 



Remaining legend same as in Figure 103. 



Pluteus II Stage 



The third pair of arms, the posterodorsal, appears in this stage. The rods 

 of these arms are complexly fenestrated. The rudiment of the dorsal arch 

 becomes discernible; it is considerably broader than in Echinarachnius parma, 

 about 1 80°. In a live larva the accumulation of red pigment at the tips of the 

 arms is visible under a microscope. When the larva reaches a length of 500— 

 600 |j.m, the transverse rods differentiate and extend from the posterodorsal 

 rods medially within the larva (Figure 139). 



Pluteus III Stage 



The preoral arms develop early in this stage; their supporting rods are 

 simple. As in Echinarachnius parma larvae, there are no ciliated epaulettes 

 and pedicellariae, but epidermal vibratile lobes are present. A gradual reduc- 

 tion of the plates forming the "basket" bottom takes place in the pluteus III 

 stage. Soon after their disintegration, resorption of the iimer transverse rods 

 occurs. The amniotic sac becomes visible, arising, usually, on the 6th- 7th 

 day of development. The larva by this time has reached a length of 800-900 

 |im (Figure 140). 



Ecology 



Larvae of S. mirabilis appear in the plankton of Vostok Bay in late July 

 when the water temperature in the surface layer is 18°C. Maximum density 

 is observed in mid-August. 



