4 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.41. 



This grouping, as well as other classifications employed throughout 

 the paper, is a purely artificial and arbitrary one. The intention is 

 merely to bring such forms together as show a certain similarity in 

 structure, and thus aid in simplifying the discussion of the modifica- 

 tions exhibited by different types. 



As might be expected, one genus may contain species that fall 

 under two different groups, as, for instance, Edriocrinus, certain spe- 

 cies of which belong to Group II and others to Group III. Even a 

 single species may have representatives referable to two different 

 groups. Thus certain specimens of Millericrinus prattii may be 

 referred to Group I, while by definition other individuals fall in 

 Group II. 



Group I. 



The group comprising those forms that do not become detached 

 from their columns, and yet, during a portion of their lives at least, 

 are capable of more or less free movement, is a very large one. As 

 a matter of fact, it is probable that the greater part of the Pelma- 

 tozoa, with the exception of the Edrioasteroidea and those forms here 

 included in Group II, are safely referable to this assemblage. This 

 is a sweeping statement, but one thoroughly justified, I believe, by 

 the evidence at hand. In referring to an eleutherozoic period during 

 the lives of the Pelmatozoa, one is meant subsequent to the free- 

 swimming larval stage, and after the formation of a column. In 

 most cases, perhaps, this period of detachment from the bottom 

 came at a comparatively early stage in the ontogeny of the animals, 

 and was followed by the reattachment of the organisms. Frequently, 

 however, the animals retained their freedom throughout life. Again^ 

 it is probable that in the lifetime of some forms at least there were 

 several alternating periods of attachment and freedom. In the case 

 of some of the Cystidea, it is possible that the animals were never 

 firmly affixed at any stage in their development. In the discussion 

 of this group the Cystidea will be dealt with first, to be followed in 

 turn by the Blastoidea and Crinoidea. 



CYSTIDEA OF GROUP I. 



It is probable that a large proportion of the Cystidea led a free or 

 semifree form of existence. Many were undoubtedly capable of 

 active movement, while others, attached or unattached at will, were 

 comparatively passive. It is but natural that eleutherozoic forms 

 should be more abundant among the Cystidea than among the 

 Crinoidea or Blastoidea, for they more nearly approximate to me 

 eleutherozoic archetype of all the Echinoderma. With the Cystidea, 

 then, in some cases, the eleutherozoic forms may be considered in the 

 light of organisms which had not yet attained true pelmatozoan 

 fixation. In other lines fixation, though acquired, was such a novel 



