NO. 184C. ON CERTAIN ELEVTHEROZOW PELMATOZOA—KIRK. 31 



such a condition of life for these forms in which we have the distal 

 portion of the stem preserved and showing no signs of permanent 

 fixation, it is surprising that the list is as long as it is. Were complete 

 columns more frequently found, our list would no doubt be. materi- 

 ally increased. In addition to those forms which from having exam- 

 ined complete individuals we know not to have been cemented or 

 rooted to the bottom, we may reasonably ascribe a similar existence 

 to many more. The reasons for such reference will be given subse- 

 quently under the discussion of the various forms. Not only are 

 many crinoids to be considered as eleutherozoic for the greater por- 

 tion of their lives, but also a far greater number are to be considered 

 as having occasional periods of freedom during which migration is 

 possible. In this category are probably to be placed the majority of 

 Cruioidea. 



General classiiication of the Crinoidea of Group I. — For convenience 

 of reference we may divide the detached stalked Crinoidea into two 

 groups and later subdivide these. On the one hand we have those 

 crinoids that for the greater part of their lives are not attached to the 

 bottom by a root or by cementation. These forms may be capable of 

 voluntary locomotion at will, without an attendant disruption of the 

 column. On the other hand we have those crinoids that are normally 

 firmly affixed to the bottom, which at irregular intervals, governed no 

 doubt by external conditions, break or are broken loose from their basal 

 attachment and in some cases swim about freely. It is obvious that 

 these two divisions are arbitrary at best, and differ in degree rather 

 than in kind. In one case after a period of attachment the crinoid 

 becomes free and thereafter remains so, barring accidental fixation. 

 In the other case the crinoid has alternating periods of freedom and 

 fixation. It is probable that in both divisions detachment becomes 

 irregularly effective in different individuals, dependent no doubt on 

 the conditions under which the particular form lives. 



The crinoids that are free for the greater part of their existence 

 may be divided according to their mode of life. Most of the types 

 may be considered as vagile or vagrant benthos. Some undoubtedly 

 were epi-plankton, while at least one genus may well be classed as a 

 true plankton. In the latter cases perhaps the first step in the 

 evolution of the forms away from the sessile stock was the assump- 

 tion of a type of existence approximating to that of a vagile benthos. 

 Gradually the exigencies of this life may be seen to have induced 

 modifications of structure either tending toward greater freedom of 

 movement or toward a better control of movement. As by far 

 the most striking example of the first type of modification we may 

 take ScypJiocrinus, hereafter discussed, the float of which has been 

 variously described as Camarocrinus and Lobolithus, not to mention 

 its reference to the Echinoidea. As an example of the second sort. 



