30 PROCEEDINGS OF THE NATIONAL MV8EUM. vol.41. 



BLASTOIDEA OF GROUP I. 



In the case of the Blastoidea, evidence relative to the maintenance 

 of an eleutherozoic habit by the stalked forms is quite limited owing 

 to the fact that specimens with the columns preserved are seldom 

 found. In the case of Pentremites, however, a considerable number 

 of individuals are laiown the columns of which are essentially 

 complete. In these specimens it is to be noted that the stems are 

 short and in the distal portion taper rapidly, apparently coming to 

 a point. Weak radicular cirri are present and extend for a short 

 distance up the column. It may be held, I think, that in this genus 

 permanent fixation by the adults did not obtain. 



Whether we may extend this conclusion so as to apply to the 

 remainder of the Blastoidea, it is impossible to say. Considering 

 the extreme type of an eleutherozoic habit maintained by certain 

 Blastoidea such as Eleutherocrinus , it would seem that there was a 

 decided tendency among the members of this class toward the assump- 

 tion of a free mode of life. This fact and the considerable acquisi- 

 tion of an eleutherozoic habit by the Pelmatozoa in general point 

 strongly to the conclusion that among the Blastoidea a detached 

 existence was largely maintained. The evidence afforded by the 

 distribution and segregation of the Blastoidea argues quite as strongly 

 in support of this conclusion as in the case of the Crinoidea, where 

 this phase of the subject will be fully discussed. 



CRINOIDEA OF GROUP I. 



To put upon a firm footing the statement elsewhere made that the 

 great majority of crinoid types have at one time or another, subse- 

 quent to the formation of a column, been free and capable of moving 

 about at will, is not an easy matter. In the very nature of things the 

 statement is incapable of absolute demonstration. The evidence in 

 support of the contention is of two sorts. In the first place we have 

 positive evidence as afforded by individuals in which the column is 

 complete and shows no signs of attachment. We also have the less 

 satisfactory evidence given by residual roots which in some instances 

 probably indicate detachment during the life of the crinoids — whether 

 voluntary or not we may but conjecture. On the other hand we 

 have the more or less indirect evidence afforded by the facts of crinoid 

 distribution and segregation. It will be unnecessary to go fully into 

 this phase of the subject. Such facts as we have may only satisfac- 

 torily be explained, I believe, on the assumption that post-larval 

 migration plays a part of considerable importance in the lives of the 

 Crinoidea. 



The number of stalked genera referable to the Crinoidea which 

 were and are truly eleutherozoic during the greater portion of their 

 lives is quite large. Inasmuch as we can only certainly postulate 



