NEW RESEARCHES /9 



NEW RESEARCHES 



On the Ontogeny of some Recent Crinoids bearing upon the 

 Phylogeny of the Flexibilia 



I have long had a suspicion that in the comatulid genus formerly called 

 Actinometra there might be possibilities of new information touching larval 

 growth, for comparison with developmental phases of the ancient crinoids, 

 more satisfactory than that obtained from Antedon, the only crinoid whose 

 larval stages have been heretofore known — a genus which, as has been re- 

 marked by Agassiz 1 and myself, 2 and later by Clark, 3 is not a typical repre- 

 sentative of the class, and hence, as Mr. Clark remarks, one of the least 

 satisfactory for purposes of phylogenetic investigation. As long ago as 1901, 

 in my memoir on Uintacrinus just cited, when discussing the possible effect 

 upon the calyx of changes due to the movements of the rectum, I said on p. 56: 



It would be a most important thing if somebody could work out the life history of 

 Actinometra, as W. B. Carpenter, Wyville Thomson, and Bury have done for Antedon. 

 There are several species which seem to be sufficiently littoral in habitat to make this practi- 

 cable. The " pentacrinoid " stage of Act. meridionalis has been seen. 



And at several times since then I have made plans for such an investiga- 

 tion which for various reasons always fell short of execution. Recently, how- 

 ever, I had the extraordinary good fortune to come into possession of a brood 

 of " pcntacrinoids " of the identical species above mentioned taken on the coast 

 of Yucatan. They form a series ranging from the sessile pre-brachial stage to 

 that in which the orals are resorbed, all permanent elements of the skeleton 

 fully established, and the stem discarded for a free existence. Upon examin- 

 ing them under a microscope it was clear that we had in these young 

 comatulids the evidence of a course of development in oral, interradial, and 

 anal structures considerably different from that seen in Antedon, and which is 

 analogous in an extraordinary degree to the condition and progressive modifi- 

 cation of these structures in the fossil Flexibilia. To accurately determine the 

 different stages exhibited by these specimens, and make drawings for such as 

 might be desired for illustration, involved a laborious investigation with tech- 

 nique in microscopy which I do not possess. But this has been accomplished by 

 my assistant, Dr. Herrick E. Wilson, with patience and skill to which the three 

 plates of figures, A, B and C, bear ample testimony, and for which I wish to 

 here record my acknowledgments. I have thought it more satisfactory to 

 reserve the description of this new material, which has a direct bearing upon 



1 Calamocrinus, Mem. Mus. Comp. Zoology, Harvard, vol. 17, no. 2, 1892, p. 52. 



'* Uintacrinus, ibid., vol. 25, no. 1, 1901, p. 38. 



3 Monograph of the Existing Crinoids, 1915, p. 331. 



