EVOLUTION OF THE ECHHSTODERMS — FELL 



475 





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Figure 10. — Sequence of cross sections of the arm in: A, biserial crinoid; B, Chinianaster; 

 C, Platasterias; D, Platanaster (Platyasterida); E, Luidia; F, Astropecten. Abbreviations: 

 1, ambulacra! (brachial) ossicle; 2, virgalium-1; 3, virgalium-2; 4, virgalium-3; 5, vir- 

 galium-4; 6, superomarginal paxilla. (Homologies of virgalia are as given in caption to 

 Fig. 8.) 



