NO. 184G. ON CERTAIN ELEUTHEROZOJC PELMATOZOA—KIRK. 127 



bottom a position once assumed would be maintained unless altered 

 by definite swimming movements of the crinoid. Slight adjustments 

 of position might at any time be secured by motion of the arms, or of 

 the cirri, where such were present. 



It has been suggested by Carpenter that the comatulids seldom 

 move about to any extent except during the breeding season. Clark 

 (1909 a) confines the free-swimming period of multibrachiate comatu- 

 lids to those stages in which a ten-armed condition obtains. During 

 this stage the young, when observed, are continually swdmming about. 

 Wlien the multibrachiate stage is assumed they are incapable of 

 swimming movement. Whether or not this be the case it is hard to 

 tell, but it seems Hkely that, in these forms at least, the adults are 

 incapable of any considerable activity. This prohibition is obviously 

 dependent upon the structure of the arms. This is of such a nature 

 as to render any considerable motion impossible. It is conceivable, 

 however, that the crinoids may crawl by employing the distal por- 

 tions of their arms. In the case of adult detached crinoids as a whole 

 there seems no physical disabihty in most cases rendermg the crinoids 

 incapable of using their arms as s^^'imming organs. It seems probable, 

 however, by the time an adult stage is reached that the crinoids wall 

 have found a favorable environment and under normal conditions 

 would be content to remain there. Detachment from the bottom 

 then need by no means indicate active swimming movements on the 

 part of the animals involved. Indeed, among the stalked detached 

 forms one may hold that the type of life led is essentially statozoic. 

 As a rule advantage is taken of their freedom probabl}^ only during 

 periods of sexual activit}^. 



Eleutherozoic conditions as indicative of progression or degeneracy 

 among the Pelmatozoa. — It should be borne in mind that the tendency 

 toward an eleutherozoic existence among the crinoids is not a rever- 

 sion, in the sense of indicating a loss of vigor in the stock, and degen- 

 eration. The freedom of the crinoid is rather an expression of a higher 

 degree of specialization along perfectly natural fines. The discarding 

 of the stem after a long period of attachment, as I see it, is a sign of 

 strength rather than weakness, as indicating a tendency on the part 

 of the organisms toward the elimination of semiuseless structures for 

 the attainment of a type of life more beneficient to the crinoids. The 

 fact that highly specialized eleutherozoic conditions do not more 

 largely obtain among the Crinoidea is due in no small part to the fact, 

 as has elsewhere been pointed out, that the animals in the main are 

 not fitted for the efficient maintenance of this type of existence. 



Among the Cystidea, on the other hand, the majority of the free 

 forms may be considered degenerate. Indeed, lack of fixation among 

 these types may be held as evidence of failure on their part to acquire 

 the statozoic habit toward which they were aiming. Such a case is 



