OX TJIK OKKJIN OF CERTAIN TYPES OF CRINOID STEMS. 



By Austin Hobart Clark, 



Assistant Curator, Division of Marine Invertebrates, U. S. National Museum. 



In a recent number of the American Naturalist" 1 brought forward 

 what appears to me to be conclusive evidence of the very close rela- 

 tionship between the Echinoidea and the Crinoidea, which two groups 

 I placed, together with the Holothuroidea, in the new sub-phylum 

 Echinodermata Heteroradiata in contradistinction to the Echinoder- 

 mata Astroradiata, which comprises the Asteroidea and the Ophi- 

 uroidea. The paper was necessarily short; only the major features 

 of the interrelationships were considered, as in a dissertation of that 

 character wealth of detail always means lack of strength; and many 

 minor points connected with the homology in whole and in part 

 between the urchins and the pelmatozoa yet remain to be elucidated. 

 One of the most important of these minor considerations is the proba- 

 ble relationship between the column of the crinoid and the central or 

 sur-anal plate of the echinoid; how may one of these structures rea- 

 sonably be derived from the other? and how may widely different 

 types of columns such as those of Edriocrinus, Phrynocrinus, Platy- 

 crinus, Metacrinus, Holopus, Baihycrinus, Calamocrinus, etc., be 

 logically reduced to a primitive common ancestor? 



First of all there is one feature which may, perhaps, require a word 

 of explanation. I have homologized the column of the crinoid with 

 the sur-anal plate of the urchins, and for this 1 have been criticized 

 by my friend. Dr. Th. Mortensen, of Copenhagen, on the ground that 

 the so-called "Palaeoechinoidea," the oldest known echinoids, lack the 

 sur-anal plate. 1 was aware of this fact at the time I wrote the paper, 

 but it did not appear to me to have any weight whatever, for iu the 

 structure of the test the " Palseoechinoidea " are in certain ways far 

 more specialized than any recent species, and, as specialization is 

 usually accompanied to a greater or lesser degree with the suppression 

 of more or less fundamental primitive structures, I assumed that, 

 although the sur-anal plate was usually retained in a more or less 

 reduced form by all recent types, there was no reason for supposing 

 that, were the recent genera t<> attain mult icolumnar ambulacra! and 



a Vol. 43, ]>. 682, November, 1 <)()<). 



Proceedings U. S. National Museum, Vol. 38— No. 1740. 



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