38 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.41. 



forms with long internodes have comparatively longer columns than 

 those with nodals situated more closely together. The reason for 

 this seems to be that a certain number of cirri by which the animal 

 is able temporarily to attach itself is essential to the welfare of the 

 crinoid. So in cases where the internodes are long there must be a 

 considerable length of column in order to bear an adequate number of 

 cirri. Where a considerable compression of nodals obtains it is 

 obvious that a much shorter column suffices. 



Specialization of cirri. — The degree of specialization shown by the 

 cirri of the recent Pentacrinidse although not as marked as in the case 

 of the fossil forms hereafter described, is nevertheless notable. It 

 will be observed that the cirri are of considerable length and are 

 equipped with terminal claws. They are quite flexible and motion 

 is readily had in a vertical direction. The junction of the cirrus with 

 the column is marked by a well developed articular face. The 

 structure is clearly indicative of adaptation to considerable freedom of 

 motion and the development of a fairly strong musculature. As has 

 been observed in the case of living Isocrinus the cirri move with fair 

 rapidity and are capable of grasping and holding tenaciously to extra- 

 neous objects. 



Conclusions relative to the recent Pentacrinidse. — From the evidence 

 as given above, it may be concluded, I believe, that the majority of 

 the recent Pentacrinidse pass the greater part of their lives as de- 

 tached organisms. It may be that in some individuals, after the 

 initial period of attachment, the animal passes through alternating 

 periods of freedom and fixation. In the majority of cases, however, 

 it is probable that prolonged fixation only obtains in the case of the 

 very young forms. Permanent attaclmaent when had by the Penta- 

 crinidse seems to be entirely by cementation. Specimens brought up 

 on cables show this type of structure.^ It is quite conceivable that 

 after a period of freedom the crinoid may well settle down and by such 

 a secretion of stereom as is to be noted in all the specimens on the 

 distal end of the column, cements itself to the bottom. While cemen- 

 tation is becoming effective, the animal could hold itself in place by 

 means of its terminal cirri. These, after cementation, are lost. 



Detached conditions among the fossil Pentacrinidse. — Having found 

 such universal freedom in the case of the living Pentacrinidse we may 

 well look for similar conditions of life among the related fossil types. 



1 Mr. A. H. Clark infonns me that the statement relative to the cementation of Isocrinus to telegraph 

 cables as reported by Agassiz and quoted by Carpenter (1884, p. 18) is erroneous. It is Mr. Clark's belief 

 that fixation by cementation never becomes effective after the primary detac'liment of the organism. It 

 may be that such is the case, but their seems to be no special reason why recementation should not occur. 

 It is obvious that those physical conditions that bring about the detachment of a crinoid are not particularly 

 conducive to its reattachment. They need by no means be prohibitive, however. There seems quite 

 positive evidence in favor of the theory of alternate periods of fixation and freedom in tlic case of the Paleo- 

 zoic Crinoidea, and if such conditions obtained here it does not seem at all improbable that we may have 

 a similar habit expressed by the modern forms. 



