462 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1962 



Figure 4. — Convergent types of development in the life histories of unrelated echinoderms. 

 A, a sea cucumber (Cucumaria). B, a brittlestar {Ophioderma). C, a crinoid {Antedon). 



evidence Caswell Grave had suggested as long ago as 1903 that a 

 yolky egg and a vitellaria, or yolk-larva, may be an ancient feature of 

 echinoderms, and that larvae such as the pluteus must be only special- 

 ized later developments. As, however, some ophiuroids with partly 

 direct development have a vestigial pluteus, this has been taken as 

 evidence that direct development is secondary and that yolk-larvae 

 are a special kind of modified form without wide significance. Until 

 last year no answer could be given to that objection, but, as will be 

 mentioned later in this discussion, the impasse now seems to be soluble. 

 Certain serious objections to the embryological theory were also 

 raised by comparative anatomy and paleontology. Fossil evidence 

 already available in 1948 implied that ophiuroids had arisen from 

 some generalized lower Paleozoic stock of sea stars, from which 

 modern starfishes must also have descended. Echinoids and holothuri- 

 ans, according to the evidence of fossils, seemed to have been derived 

 from ancient pelmatozoans, and certainly could not be related to the 

 star-shaped groups in the strange way which the embryological evi- 

 dence implied. Even at that date no paleontologist could have 

 doubted that asteroids and ophiuroids arose from a common ancestry, 

 yet the embryological evidence, still cited by one author of a textbook 

 as late as 1955, implied a closer afl^ity between ophiuroids and echi- 

 noids. Obviously more definite evidence was needed. 



