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ANNUAL REPORT SMITHSONIAN INSTITUTION, 1962 



by which it can be demonstrated that luidiid skeletal structure had 

 already appeared in the lower Ordovician sea star Platanaster. Thus, 

 though Luidiidae have until now been thought to be a modern group 

 without fossil representatives, on the contrary they are now seen to be 

 truly living fossils of the antiquity of Platanasteridae. Edward 

 Llliuyd, who first recognized that featherstars are surviving sea lilies 

 in a paper published in 1699, is honored by the Latinized version 

 of his name, Luidius, from which Luidia is derived. How justly 

 named that genus is ! To return to my subject, however, it was obvi- 

 ous that Luidia could not be directly related to somasteroids, for, 

 as seen in figure 6, H, the cross section of the arm shows a very 

 deep furrow excavated on the lower surface, bounded by the ambula- 

 cral ossicles, and housing the tube feet. All starfishes have this fea- 

 ture, called the ambulacral groove. It is really the diagnostic char- 

 acter of Asteroidea. It is produced by the erection of the ambulacral 

 ossicles, which stand upon the adambulacral ossicles, and it serves 

 the gross feeding which typifies starfishes. Somasteroid fossils did 

 not exhibit this structure, for the ambulacral ossicles are recumbent. 

 Again, as seen in figure 6, I, the body of Luidia differs considerably 

 in form from that of a somasteroid, for the arms are long and strap- 

 like, as in many modern starfishes. 



We may now consolidate the data so far. Figure 7 sets out in 

 simplified schematic form the difi^erent patterns of growth gradient 

 fields so far established in the various Asterozoa. On the right is the 

 pattern typical of all asteroids except Luidiidae; it is dominated 

 by longitudinal gradients. In the center is the pattern seen in Luidi- 

 idae and all Ophiuroidea, with dominant longitudinal gradients 

 intersected by short transverse gradients. On the left is the inferred 

 gradient field of a chinianasterid somasteroid, in which dominant 



B 



Figure 7. — Growth-gradient fields in Asterozoa. A, somasteroids (found also in pinnulate 

 crinoids). B, Platyasterida and Ophiuroidea. C, Asteroidea, other than Platyasterida. 



