NO. 1846. ON CERTAIN ELEUTHEROZOIC PELMATOZOA—KIRK. 50 



is restricted to an isolated group of organisms. Rather, such depar- 

 tures are found to have taken phice again and again in a more or less 

 marked manner, not only in contemporaneous, but in antecedent and 

 subsequent forms. As regards this particular modification, or 

 rather habit, we fuid evidence that indicates its appearance from the 

 earliest times to the present, and among most diverse types. 



In the case of the majority of the crinoids described above, free- 

 dom may be held to obtain during a considerable part of the lives of 

 the animals, at least throughout the greater portion of the adult 

 stages. Among the Crinoidea as a whole we shall consider that 

 detachment becomes effective for variable periods, and perhaps 

 alternates with times of fixation. This is sufficiently inclusive, I 

 think, to cover any case. It is obvious that it is among types where 

 detachment becomes irregularly effective that we have the progeni- 

 tors of those forms which constantly assume and maintain an eleu- 

 therozoic habit. It is no doubt among these forms again that we 

 have the ancestors of those crinoids which in the aggregate make up 

 Group II. No line, except of the most arbitrary sort, may be drawn 

 between the types in which detachment is the exceptional condition 

 and those among which it is the rule. In all we observe to a greater or 

 less extent the operation of the same tendency becoming progressively 

 more effective. 



In the following discussion evidence of detachment by the crinoids, 

 whether structural or of whatsoever nature, is given. The matter 

 relative to crinoid segregation and migration has a most important 

 bearing on the question, besides indicating in part the reasons for 

 the assumption of such a type of existence. Such evidence as is sub- 

 mitted is more or less general in that such facts as are adduced from 

 specific types may be held to apply with greater or less force to the 

 remainder of the stalked Crinoidea. In the case of the structural 

 evidence again the material in most instances is incapable of exact 

 identification and can only be credited in an indefinite way to the 

 then existing crinoid fauna. 



The evidence relative to these crinoids that are universally sup- 

 posed to be firmly affixed from the time the column is formed and in 

 which an eleutherozoic habit becomes but irregularly effective, is of 

 necessity not conclusive. Such facts as are available, however, seem 

 to make the conclusion that sporadic and perhaps long-continued 

 periods of freedom are of comparatively common occurrence, the 

 only feasible one. It is these post-larval periods of detachment that 

 probably have had more to do with the segregation of the Crmoidea 

 than the aimless wanderings of their ciliated larvae, and it is no doubt 

 m no small part due to the same factor that the Crinoidea owe their 

 perpetuation as a virile stock. 



