184 



development, the first signs of resorption of the larval skeleton becomes 

 evident. The basal plate in "the basket" disintegrates. The quantity of pigment 

 increases but only slightly. A fairly outstanding feature of the larva of this 

 stage is its change in color when placed in 70° alcohol. The red in the 

 pigment granules disappears and is replaced by green, which evenly colors 

 the entire larval body (Figure 137). The length of the larva is 700-800 |a.m. 



Ecology 



Pluteus I stage begins to appear in the plankton of Peter the Great Bay 

 in early July, when the water temperature in the surface layer is 16.5°C. 

 Under conditions favorable for spawning, they may be found even in the 

 second ten-day period of June. The maximum density of pluteus I stage larvae 

 is observed in mid- July. The first individuals of settling juveniles are seen 

 after July 10. 



According to Costello and coworkers (1957), spawning of £. parma along 

 the Atlantic coast of America (Woods Hole, Massachusetts) is observed fi"om 

 late February to mid-August. 



SCAPHECHINUS MIRABILIS (AG ASSIZ) 



(Dendrasteridae) 



Mature eggs have a secondary envelope with inclusions of red pigment. The 

 eggs are 90—100 |im in diameter and the envelope 40 |a.m thick. Free- 

 swimming blastulae were found 13 hrs after fertilization in cultures at a 

 temperature of 20-21 °C. Four hours later gastrulation was observed, and in 

 the 20th hr the first triradiate calcite spicules appeared in the gastrulae. The 

 early plutei developed 34 hrs after fertilization. 



Pluteus I Stage 



The larval body is transparent, shortened with a broad and flat base, and 

 slightly tapering. Two pairs of arms are present; anterolateral and postoral. 

 The structure of the rods supporting these arms is the same as in Echinarachnius 

 parma. The inner transverse rods join medially but unlike in E. parma do not 

 bear spines. The basal and secondary basal rods fuse in pairs. Each rod 

 thereby enlarges somewhat to form a small plate or lobe and basal and 

 secondary basal rods merge at the place of contact of these plates. Two large 

 plates develop at the base of the larval body, bearing numerous spines on the 

 lower margin and several small fenestrations. Thus, the "basket" in S. mirabilis 

 comprises two containing parts. The height of the "basket" base is three- 

 fourths its width. At this stage, the length of the larva is 200-400 |j.m (Figure 

 138). 



