188 



Figure 140: Scaphechinus mirabilis. Pluteus III Stage, 

 as — amniotic sac; Remaining legend same as in Figure 109. 



Pluteus I Stage 



The structure is identical in S. griseus to that of the pluteus of E. parma. 

 The basic difference in the pattern of the connection of the basal and second- 

 ary basal rods. In S. griseus these are not fused and the true "basket," 

 characteristic of the larvae of sand dollars, is not formed. The ends of basal 

 rods have a similar thickening from which fairly long spines arise. These 

 spines are somewhat smaller in secondary basal rods. The rods interlock 

 through spines which fit into depressions formed by them on the opposite 

 pair. True fusion does not occur. The structure resuhing from such an ar- 

 rangement may, at best, be termed a "pseudobasket". The width of the 

 "pseudobasket" base is three-fourths of its height. The length of pluteus I does 

 not exceed 400 jim (Figure 141). 



Pluteus II Stage 



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

 postoral, they have complex fenestrated rods. Simultaneous with them, the 

 dorsal arch develops which, as in Echinarachnius parma, is about 120°. The 

 larval body remains transparent, devoid of pigment granules. By the end of 

 stage II, the larva has increased in length up to 400 )a,m (Figure 142). 



