192 



Pluteus II stage 



Five days after fertilization the third pair of arms, the posterodorsal, 

 appears. The rods supporting this pair of arms are complexly perforated. 

 Before the rudiments of the posterodorsal rods emerge, the connection be- 

 tween the basal and secondary basal rods disintegrates and the secondary 

 basal rods are reduced; however, the lower transverse septum is retained. The 

 developing posterodorsal rods bear two basal processes, the larger of which 

 fuses with the secondary basal rod while the smaller, hook-shaped, attaches 

 itself to the aterolateral rod. After emergence of the lower inner transverse 

 rods, diverging from the base of the secondary basal rods, the connection 

 between the basal and secondary basal rods totally disappears. By the time 

 of resorption of this connection, the smaller process of the posterodorsal rod 

 has straightened somewhat and runs parallel to the anterolateral rod. On the 

 side opposite to it, a new process begins to grow, which is directed below. 

 The dorsal arch appears at this time. 



As soon as the posterodorsal arms attain the size of the postoral arms, the 

 ends of the dorsal arch begin to rapidly extend upward, giving rise to rods 

 for a new pair of arms, the preoral. Simultaneous with their development, the 

 lateral processes at the base of the aboral rod enlarge. The length of the 

 pluteus II stage is 900-1,000 |im (Figure 145). 



Pluteus III stage 



On the 7th -8th day of development, when the basal ends of the aboral 

 rod reach the base of the posterodorsal rods, a fifth pair of arms, the postero- 

 lateral, appear. The calcification centers of this pair appear from the outer 

 side somewhat below the lower inner transverse rods. The simple rods grow- 

 ing from these centers fuse with the basal ends of the aboral rod. The pos- 

 terolateral arms situated perpendicular to the anterolateral arms, together with 

 the aboral rod constitute a single structural unit which serves as a support for 

 the entire larval body. On the right, the arced rod connects the basal end of 

 the aboral rod with the posterodorsal rod. Probably the aboral rod develops 

 as a result of the enlargement of the lower lateral process of the posterodorsal 

 rod. A small perforated plate develops on the outer side of the postoral rod. 

 On the left side, likewise, a perforated plate appears on the postoral rod but 

 there is no connection with the aboral and posterodorsal rods. At this time, 

 on the lower side of the dorsal arch, a pair of processes appears on each side 

 of the esophagus. The upper processes remain short while the lower ones 

 grow rapidly. By the 10th- 11th day, this lower pair has modified into rods 

 of the anterodorsal arms. The short processes of the basal arch serve as an 

 additional support for them from below. With the appearance of the sixth pair 

 of arms, formation of the larval skeleton is complete. The larvae of E. 



