476 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1962 



various grades of asteroids below. Space will not permit detailed 

 analysis, but note the gradual conversion of originally slender virgalia 

 into massive blocklike elements in the later grades, and the simul- 

 taneous erection of the ambulacral ossicles upon the proximal virgalia 

 (or adambulacrals, as they now become) to produce the elevated in- 

 vaginated furrow of asteroids. Although many details, including all 

 the soft parts, have had to be omitted from this condensed account, I 

 think it will by now be clear how the asteroids can be traced back, 

 without any sharp break, to the ancient lower Paleozoic somasteroids. 

 We can, I believe, trace them still farther back, but first of all, what of 

 the Ophiuroidea? 



PINNATE STRUCTURE IN BRITTLESTARS 



It would be tedious and unnecessary to attempt here to trace the 

 ophiuroids backward in time in such detail, and of necessity this part 

 of the topic must be cut to its bare essentials. Glance again at figure 

 5 to refresh your memory of the essentially pinnate structure of Ordo- 

 vician somasteroids. Since we have ascertained that the ambu- 

 lacral ossicles alternate near the tip of the arm, and in the young 

 stages of Ghinianasfer^ it follows from Jackson's so-called "law of 

 localized stages" that the opposite condition of the ambulacral ossicles 

 must have been derived from an original alternating condition. This 

 alternating condition was retained in Archegonaster, in some Paleo- 

 zoic asteroids, and in some of the Paleozoic ophiuroids. One very 

 important point emerges. It has been shown how asteroids have 

 arisen from a former pinnate grade of organization, represented by 

 somasteroids, and that the most primitive asteroids preserve pinnate 

 features in the arm. Now, if ophiuroids, as we suspect, arose from a 

 similar ancestry, they too should exhibit evidence of pinnate struc- 

 ture. A considerable number of genera were accordingly dissected, in 

 search of such features, and the results are illustrated by the repre- 

 sentative genera shown in figure 11. Except for figure 11, E, which 

 shows an Ordovician genus with alternating ambulacral ossicles, all 

 illustrations are of surviving genera. The pinnate structure is self- 

 evident, and it is indeed strange that so little attention has been paid 

 to such evidence hitherto, almost the entire classification having been 

 based on superficial characters not requiring dissection. The condi- 

 tion D, in the middle, shows the final simplification, as seen in the vast 

 majority of surviving genera. Yet even in this simplified state, the 

 homologies of the ossicles are easy to determine. In fact, the entire 

 skeleton in ophiuroids can now be equated with known structures in 

 somasteroids. 



