32 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.41. 



the whorls of cirri of the Pentacrinidse and other forms may be cited. 

 The attachment of certain Crinoidea to floating bodies by which 

 they are carried from place to place might perhaps be expected of the 

 very young stages, but that this epi-planktonic existence should 

 largely be assumed by certain adult forms, as we know it to be, is 

 surprising, to say the least. 



The vagile benthos. — In discussing the group of vagile benthos it 

 may be well to deal first with those forms which from structural 

 evidence we know to have passed a very considerable portion of their 

 lives as detached organisms. This evidence consists either of the 

 possession of specialized organs adapted to the maintenance of an 

 eleutherozoic existence or a lack of special organs fitted for the per- 

 manent attachment of the crinoid. 



Although permanent fixation does not become effective among 

 these types, it is obvious that the ability to attach themselves tempo- 

 rarily would be of considerable advantage to the crinoids. Even 

 among those forms referred to Group II active swimming periods 

 are apparently of irregular occurrence and of but brief duration. 

 Having located in some favorable spot, the crinoid needs a slight 

 anchorage to maintain its position, as it tends to be changed by 

 current or wave action. To meet such conditions we find various 

 modifications, and a rough grouping may be made on the basis of 

 similarity of such structures. 



Types of temporary fixation among the vagile henthos. -^Thvee types 

 of temporary attachment may be recognized. In the first place, the 

 crinoid may hold onto the bottom by a looping of the distal portion 

 of the column about some stationary object. Secondly, there may 

 be no true attachment, but the presence of a distal knob or anchor 

 may serve to ballast the crinoid and maintain it in approximately 

 the same position. Finally, as is the case more commonly among 

 the Mesozoic and later forms, we find that attachment to other 

 objects is effected by means of prehensile cirri borne upon the column. 

 The order here given indicates in a broad way the general degree 

 of complexity of the types of structure involved. The second type , 

 of fixation, if such it may be called, is quite out of the regular line 

 of evolution, and represents both simple and complex conditions. 

 For instance, the terminal knobs elsewhere described as occurring 

 either on Dolatocrinus or Megistocrinus columns probably repre- 

 sents secretion of stereom induced by and following a violent disruption 

 of the column. On the other hand the grapnel of Ancyrocrinus is a 

 highly specialized organ resulting from the modification of radicular 

 cirri. In the treatment of these crinoids the forms will be discussed as 

 grouped above. The order will be changed, however, the most com- 

 plex types being taken first. This is done in order immediately to 

 make available the evidence afforded by the living Crinoidea. 



