EVOLUTION OF THE ECHINODERMS — FELL 



477 



Figure 11. — Archaic brittlestars (Ophiuroidea), showing persistence of pinnate structure 

 of somasteroid type, and presence of reproductive and alimentary organs in the arms; for 

 further explanation see text. A, Asteronyx. B, Trichas er. C, Jsirophyton. D, 

 modern type of brittlestar (Opkiomastus) for comparison; E, Eophiura. F-J, Ophiocanops 

 (Oegophiurida). Abbreviations: A, ambulacra! ossicles (fusing to form vertebrae); C, 

 caecum of stomach extending into arm; M, madreporite; V, ventral arm plate; 1, first 

 virgalium (V-1, sublateral plate); 2, second virgalium (V-2, lateral plate); 3, third 

 virgalium (V-3, primary radiole); 4, presumed fourth virgalium (V-4). In Astrophyton 

 the ambulacra! elements are concealed beneath the second virgalia. F shows the dislc in 

 ventral aspect, G the arm skeleton in dorsal aspect, H the arm in ioto in dorsal aspect, I 

 the disc in dorsal aspect, J the arm skeleton in ventral aspect, all in Ophiocanops. 



