THE STRUCTURE AND RELATIONSHIPS OF CERTAIN 

 ELEUTIIEROZOIC PEL:MAT0Z0A. 



By Edwin Kirk, 



Of the United States Geological Survey. 



INTRODUCTION. 



As a result of observations covering several years both in the 

 laboratory and in the field, it has become increasingly evident that 

 many of the commonly accepted ideas relative to the habits of the 

 Pelmatozoa are in need of a considerable amount of revision. I 

 have been fortunate in having the extensive collections of IMr. Frank 

 Springer and of the United States National Museum placed at my 

 disposal for study, and the observations made upon this material 

 and embodied in the present paper go far toward establishing the 

 conclusions here set forth. I wish here to express my thanks to Mr. 

 Springer and to Dr. R. S. Bassler for the many favors extended to 

 me during the preparation of this paper, and I am further indebted 

 to Dr. Bassler for help received in the preparation of a number of the 

 illustrations here used. ^Ir. Austin Hobart Clark, of the United 

 States National Museum, has reviewed my manuscript and has very 

 kindl)^ offered a number of valuable suggestions. Advantage has 

 been taken of these in several instances, and in all cases are accred- 

 ited to him where used. 



The division of the Echinoderma into two grades, Pelmatozoa 

 and Eleutherozoa, clearly reflects the common concept of the 

 Echinoderms as consisting of freely moving forms on the one hand 

 and statozoic types on the other. As a matter of fact no sharp line 

 of demarcation may be drawn on the basis of the relative freedom of 

 the animals, for the Pelmatozoa, or supposedly statozoic echino- 

 derms, show this character to but an indifl'erent degree. It is prob- 

 able indeed that we may hold the Edrioasteroidea alone as con- 

 sistently affixed types. 



SYSTEMATIC DISCUSSION. 



In the following paper an attempt is made to bring together some 

 of the more important points relating to certain Pelmatozoa that for 

 a part of their lives lead a free or semifree existence. Other forms 



Proceedings U. S. National Museum, Vol. 41— No. 1846. 

 94428°— Proc.N.M.voI.41— 11 1 1 



