198 



sarsi the cilia are well developed but the fertilization membrane is not shed 

 even though the cilia emerges from it. 



Gastrula 



Formation of the gastrula takes from 9-12 hrs in Amphioplus abditus 

 (Hendler, 1977) and Ophiothrix oerstedi (Mladenov, 1979) to 40-48 hrs in 

 Ophiocoma nigra (Narasimhamurti, 1933). In Ophiura sarsi the gastrula 

 develops in one day. 



In Ophiopholis aculeata (Olsen, 1942) and Ophiura sarsi the blastula 

 elongates before gastrulation along the animal- vegetal axis. 



As soon as the vegetal pole of the blastula flattens, emigration of the 

 primary mesenchymal cells begins here (Figure 148). Then an invagination 

 appears, w^hich forms the archenteron. Simultaneously, tapering of the ani- 

 mal pole region occurs where the so-called crest is formed; the latter consists 

 of vacuolated cells. Meanwhile the vegetal pole remains flat. At the same 

 time, the secondary mesenchymal cells begin to accumulate at the base of 

 the archenteron. Soon a bilobate coelomic sac develops from the archenteron, 

 which later divides into the right and left coeloms. The archenteron bends in 

 a ventral direction and touches the ectoderm to form an invagination in this 

 site. A rudiment of the esophagus is formed. After the ectoderm and the 

 bottom of the archenteron rupture, a gut is formed. The blastopore has now 

 become the anal opening. 



Until the formation of the oral opening, a dense ridge formed from the 

 ciliated ectodermal cells is situated slightly above the blastopore. This ridge 

 girdles the entire gastrula. It delineates the fiiture preoral field of the larva 

 (Figures 149, 150). Soon after the formation of the ciliated ridge, the crest, 

 in species where it is present, reduces. Under the ridge, along the edges of 

 the archenteron, in the accumulation of mesenchymal cells, spicules become 

 differentiated, from which the basal rods of the larval skeleton will develop 

 later. The embryo gradually acquires a triangular shape and changes over to 

 the prism stage. 



Prism Stage 



This stage is reached by the embryo after 16 hrs of development in 

 Amphioplus abditus (Hendler, 1977), after 48 hrs in Ophiopholis aculeata 

 (Olsen, 1942), and after 2 days in Ophiothrix fragilis (McBride, 1907). The 

 prism forms on the 4th day of development in Ophiocoma nigra 

 (Narasimhamurti, 1933) and in Ophiura sarsi. 



At this stage, the embryo gradually acquires the shape typical of the 

 ophiopluteus. By this time, the blastopore has already shifted to the ventral 

 side, although division of the gut into sections has yet to occur. The larva is 



