200 



Figure 150: Ophiopholis aculeata. 



(Fewkes, 1893). Gastrula. General view. 



bl — blastopore; cr — ciliated ridge; 



cs — crest. 



Pluteus I stage 



Figure 151: Ophiura sarsi. 



Prism stage. Optical section. 



g — gut; sp — tetraradiate spicule. 



Skeleton : In view of the fact that already at the prism stage there are in 

 some species rudiments of two pairs of arms, as in Ophiothrix fragilis (McBride, 

 1907), or three pairs of arms in others, as in Ophiura sarsi, and Olsen (1942) 

 describes a triradiate spicule in the prism stage and a tetraradiate spicule in 

 the early pluteus in Ophiopholis aculeata, the division of the developing 

 larvae into stages based on morphological characters is somewhat difficult. A 

 comparison of the morphological features of plutei of various species of 

 brittle stars based on the data of many authors (Fewkes, 1893; McBride, 1907; 

 Mortensen, 1921, 1931, 1937, 1938; Narasimhamurti, 1933; Olsen, 1942) 



revealed that the first to develop are the 

 basal and wideset, long posterolateral rods. 

 The remaining arms, that is, the anterolateral 

 and postoral, develop somewhat later, de- 

 spite the fact that their rudiments are al- 

 ready present (Figure 152). The time inter- 

 val between the growth of processes giving 

 rise to the anterolateral and postoral rods 

 may be quite substantial, from a few days 

 to a few weeks. Thus, Ophiura sarsi needs 

 one week to develop from the prism stage 

 to the complete formation of the postoral 

 rods, Ophiothrix fragilis requires three days 

 (McBride, 1907), and Ophiopholis aculeata 

 10 days (Olsen, 1942). Since already in the 

 early pluteus there are rudiments of the three 



Figure 



aculeata 



152: Ophiopholis 



(Fewkes, 1893). 



Early pluteus. General view. 



al — anterolateral arms; pi — posterolateral 



arms. 



