EVOLUTION OF THE ECHESTODERMS — FELL 



479 



' isojr 



Figure 12. — Ophiocanops fugiens (Oegophiurida), cross sections of the arm showing ordinal 

 characters. Abbreviations: 1, vertebra; 2, lateral plate; 3, ventral tegumentary muscle; 

 4, dorsal coelom; 5, ventral coelom (ambulacral vessel possibly destroyed, lost during 

 sectioning, apparently very small if present); 6, dorsal caecum of stomach; 7, dorsal 

 integument; 8, radial nerve; 9, subneural ("epineural") canal; 10, dorsolateral muscle; 

 11, ventrolateral muscle; 12, ovary and part of oviduct. (Drawings from sections cut 

 by A. G. Clark, from specimen in Copenhagen University Museum.) 



leagues C. W. Wright and Georges Ubaghs, and both at once informed 

 me that they concur with my conckisions, and that Ophiocanops is 

 indeed another example of the unsuspected persistence of archaic 

 Paleozoic groups. 



Figure 13 illustrates cross sections of the arm in three major groups 

 of ophiuroids ; A shows one of the lower Paleozoic Stenurida, such as 

 Eophiura (also shown in fig. 11, E, in ventral aspect) . In figure 13, B, 

 is a typical oegophiurid, and comparison with figure 12 indicates 

 that the ventral groove (labeled 4) would be sealed over by muscle, 

 not open (as hitherto supposed). Figure 13, C, shows a typical 

 member of the Ophiurida, the last order to appear in the fossil 

 sequence, and the one to which all modern forms are assigned (save 

 for Ophiocanops) . 



ALL ASTEROZOA FUNDAMENTALLY PINNATE 



Returning once again to our main theme, which is the phylogeny of 

 the echinoderms, it is now to be stated that all known star-shaped 

 echinoderms are fundamentally pinnate forms, and can be traced 

 back to somasteroid ancestors. A detailed analysis of the inferred 

 evolution sequence is given in another paper (Fell, in press). It fol- 

 lows that the apparent differences between the pluteus larva of 



