199 



Figure 149: Ophiothrix fragilis (McBride, 1907). 

 Gastrula. 

 commencement of gastrulation; B — formation of 



c — coelom; cr — ciliated ridge; cs — crest. 



coelomic sacs 



flattened dorsoventrally. The lateral margins of the ridge are stretched and 

 slightly upcurved. The ridge itself transforms into a ciliated band. Spicules, 

 already formed in the gastrula, are now modified into skeletal rods. Both 

 McBride (1907) in Ophiothrix fragilis and Olsen (1942) in Ophiopholis aculeata 

 have demonstrated the initiation of triradiate spicules in the gastrula stage, 

 which now give rise to three pairs of rods diverging from a common center. 

 A study of the development in Ophiura sarsi established the presence of 

 tetraradiate spicules. At the prism stage, only the upper and lower rays extend 

 into rods (Figure 151). 



Hendler's (1978) studies on the development of Amphioplus abditus showed 

 that after 24 hrs of development triradiate spicules formed in the embryo, 

 which after 5 hrs became tetraradiate. Examination of the structure of these 

 spicules under polarized light revealed differences in the crystal lattice of 

 individual spicular ends. Based on these results, Hendler concluded that the 

 first spicules do not have a single center of crystallization but are a complex 

 compound of different rods originating from different centers of crystalliza- 

 tion. Such a state is possibly associated with the peculiarities of disposition 

 of the spiculogenic cells. 



