126 PROCEEDINGS OF TEE NATIONAL MUSEUM. vol. 41. 



organisms. Even more important is the bearing such eleutherozoic 

 conditions have on the minor movements of the animals, as evidenced 

 by their segregation. It seems highly probable that the characteristic 

 colonial habit assumed by the Pelmatozoa is due in large part to the 

 selective migration of the echinoderms in their post-larval stages of 

 development. This is a highly important topic and one that will be 

 discussed at length in a subsequent paper. 



Types of life led hy the eleutherozoic Pelmatozoa. — A comparative 

 study of eleutherozoic Pelmatozoa as regards their mode of life 

 consequent upon the assumption of this form of existence is one of 

 considerable interest. For purposes of convenience we may recog- 

 nize three groups of eleutherozoic Pelmatozoa — those forms that 

 constantly remain upon the bottom; those that are capable of 

 swimming, but either temporarily attach themselves to the bottom 

 or crawl; and, finally, those forms that are truly pelagic. 



All of the stalked Pelmatozoa, with the exception of Scyphocrinus 

 and certain Pentacrinus, may be held as vagile benthos. Among 

 these benthos an erect position is almost universally maintained. 

 The only known exceptions are to be found among the Cystidea as 

 evidenced by such types as Pleurocystis and Anomalocystis. Among 

 those forms where the stem has been entirely eliminated or but a few 

 ossicles remam, movement is effected either by swimming or by 

 crawling upon the bottom in an inverted position. Among the swim- 

 ming types are to be found the most highly specialized eleutherozoic 

 Crinoidea, such as Uintacrinus. These may be held as true plankton 

 in their adult stages, inasmuch as they constantly maintain a pelagic 

 existence by virtue of their own swimming movements. 



It may safely be assumed that the stalked eleutherozoic Crinoidea 

 spent but a small part of their time in motion. Having reached a 

 favorable spot the natural tendency would be to remain there. It 

 may be argued that under these conditions permanent fixation would 

 become a necessity. This does not follow, however. In the case of 

 cirriferous individuals temporary fixation is no doubt assumed by the 

 clasping of some projection on the sea bottom with the cirri. Those 

 forms with prehensile columns could loop the distal portion of the 

 column about some stationary object. Ancyrocrinus, with its grap- 

 nel, might anchor almost anywhere. That even such types of fixation 

 were unnecessary seems to be indicated by those columns above 

 mentioned in which the distal extremity is simply rounded off into a 

 knob, and no cirri are present. Such a knob might, indeed, be held as 

 a sort of balancing organ, but when we consider the comparatively 

 small size of the knob, and its lightness, compared with the size and 

 weight of the column, this argument loses its force. It seems prob- 

 able that the weight of the column alone was sufficient ballast for the 

 crinoid, and that in the comparatively untroubled waters at the sea 



