146 



number of cells of the digestive tract in the prism stage is about 100 (Burke, 

 1980). 



On both sides of the larva, at the level of the esophagus, two tubercles 

 form — the rudiments of the first pair of arms. The entire ciliary cover 

 gradually reduces to a single ciliated band running along the margin of the 

 preoral depression and the rudiments of the arms. The ectoderm under the 

 ciliated band is thick and the cells of the ciliated epithelium are cylindrical. 



The triradiate spicules formed in the gastrula stage now increase in size 

 and each ray transforms into a rod of the larval skeleton (Figure 102). The 

 coelomic sac enlarges somewhat but is still not differentiated. The rudiments 

 of the arms gradually enlarge, and the larva acquires the shape typical of the 

 early pluteus. 



Pluteus I stage 



In sand dollars, larval development up to the pluteus I stage proceeds 

 rapidly, during only 20 hrs in Astriclypeus manni (Onoda, 1938) and 34 to 36 

 hrs in Echinarachnius parma, Scaphechinus mirabilis and Scaphechinus griseus 

 (Kryuchkova, 1977). On an average, the pluteus I stage is attained by the 

 2nd-3rd day (McBride, 1914; Aiyar, 1936; Onoda, 1936, 1938). While inves- 



Figure 102: Prism stage in Scaphechinus mirabilis. 

 ar — ^anterolateral rods; b — basal rods; po — postoral rods; sb — secondary basal rods. 



