154 



1903). The average time requirement for attaining the pluteus III stage is 12— 

 15 days from the commencement of development. 



As in the previous stage, the maximum changes are observed in the larval 

 skeleton and coelom. 



Skeleton : In most species the basal part of the larval body is broadened; 

 sometimes a transverse rod of very complex structure is situated here. A 

 fourth pair of arms, the preoral, develops in which the rods are a continuation 

 of the ends of the dorsal arch. These rods are always simple. In some heart 

 urchins, anterior to the preoral arms on the dorsal side, another pair of arms, 

 the anterodorsal, develops, whose rods initially arise as processes of the 

 dorsal arch. These, like the preoral rods, are simple in shape. In other species 

 yet another pair of arms, the posterolateral, emerges, whose rods originate 

 from the upper ends of the arcuate rods serving as the base of the aboral rod 

 (Figure 109). 



Feeding : After the appearance of the preoral arms, the system of ciliated 

 bands in the peristomial field becomes complicated. The ventral loop of the 

 anterodorsal band no longer participates in filtration of feed particles from the 

 water. It is replaced by a preoral transverse band. An adoral band likewise 

 develops. The water currents created by these bands become complex. 



Enlargement of the oral opening, priorly effected by the posterior dilater 

 muscles, is now enhanced by a pair of anterior muscles, the dilaters, situated 

 between the body wall in the region of the anterolateral arms and the preoral 

 transverse band. The rods of the preoral arms pass through these muscles 

 (Strathmann, 1971). 



According to Burke (1981), at this stage in the pluteus of Dendraster 

 excentricus the esophagus is surrounded by muscle fibers, which are circular 

 in the upper part and longitudinal in the lower. The stomach sphincters 

 consist of myoepithelial cells with transversely striated fibers. The stomach 

 wall is lined with secretory cells and cells absorbing and storing nutrients. 

 The intestinal wall is made of epithelial nonspecialized cells. 



Respiration, transport of metabolites, and excretion : At this stage, the 

 coelom of the right side remains unchanged. The left coelom enlarges, par- 

 ticularly the hydrocoel, in which five primary protuberances appear. 



Respiration : The ciliated band enlarges with the increase in the number 

 of arms. To support the larval body in water and to ensure locomotion at this 

 stage, in some species additional ciliary fields develop — the epaulettes, which 

 may comprise several pairs. Sometimes they are so closely disposed to one 

 another that an additional annular ciliated band is formed (Figure 110). The 

 length of the cilia on the epaulettes may vary in different species of sea 



