191 



Pluteus I stage 



In the early pluteus the postoral arms, which are complexly perforated, 

 are twice as long as the anterolateral arms represented by simple rods. The 

 aboral rod is present as a transverse rod with a pair of processes. Short basal 

 rods cross at the lower part of the larva. In shape, the larva initially resembles 

 a triangle but later, after the upper transverse rods appear, does not. These 

 rods fuse with each other and with the distal ends of the postoral rods. The 

 secondary basal rods diverge below from the base of the anterolateral rods. 

 These are likewise connected with each other by a short transverse rod formed 

 through the fiision of the lower transverse rods. These processes arise in the 

 secondary basal rods slightly above their distal ends. The secondary basal 

 rods are almost half as long as the basal and hence the basal part of the larval 

 body tapers slightly. Between each pair of these rods lies a transverse septum, 

 which forms above the lower inner transverse rods. The aboral rod basally 

 reaches the place of connection of the basal and secondary basal rods. On 

 the 3rd— 4th day of development, the aboral rod is equal in size to the 

 anterolateral arms. The length of pluteus I is twice that of the larvae of other 

 sea urchins, reaching 500—600 )j.m (Figure 144). 



Figure 144: Echinocardium cordatum. Pluteus I stage. 



asp — aboral spicule. 



Remaining legend same as in Figure 103. 



