120 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.41. 



the Echinoderms than is generally assumed, as is evidenced b}^ the 

 great numbers of pentacrinoid larvse that are to be found in imme- 

 diate proximity to the adults. Indeed, it has frequently been 

 observed that these larvae are attached to the arms and cirri of the 

 mature crinoids. 



Those types that have assumed a pelagic habit, such as Uintacrinus 

 and Marsujntes express the highest degree of specialization known 

 among the Pelmatozoa, in search of easy conditions of existence. 

 Among all the Echinoderma they may be noted as having most 

 nearly reached optimum conditions of life, so far as the struggle for 

 existence is concerned. As has been pointed out by Brooks (1899), 

 pelagic life is the easiest of all types of existence. The life of such 

 epi-plankton as Pentacrinus is far less favorable in that msmy indi- 

 viduals may cling to one bit of floating wood and thus materially cut 

 one another's food supply. 



Causes for reattachment of Pelmatozoa. — Despite the advantages of a 

 detached mode of life we have considerable evidence showing that 

 re-fixation fi'equently becomes effective subsequent to a period of 

 freedom. Indeed, it seems highly probable that seldom if ever has 

 a crinoid passed its entire life rooted to one spot. The reasons for 

 the nonmaintenance of an eleutherozoic habit by many crinoids 

 where disruption of the column has been caused by physical agency, 

 is probably due in large part to the fact that in these organisms the 

 tendency toward fixation is still a potent force. It must be remem- 

 bered that even in the detached forms no great activity is main- 

 tained by the Crinoidea. Carpenter states that among the Comatulse, 

 for instance, the crinoids do not swim about actively except at the 

 periods of reproduction. Again, it may well be that only those 

 forms maintain an eleutherozoic type of existence which fi*om their 

 structure are capable of effecting a fan* degree of movement. This 

 may either be brought about through the acquisition of some buoyant 

 structures or suitable modification of the arms to enable them to 

 function as swimming or ambulatory appendages. In most cases it 

 would seem that the nature of the arms is the determining factor as 

 regards the maintenance of such an existence. Were crinoids to 

 become detached that are but poorly equipped to live as eleutherozoic 

 types, what is more natural than that they should asain become 

 affixed ? 



Evidences of the assumption of a detached existence. — Naturally, the 

 most obvious indication of the maintenance of an eleutherozoic exist- 

 ence by a crinoid, is the lack of a column or pedicle. As has been 

 shown above, however, a number of stalked forms were quite as free 

 in their habits as some of the non-stalked. In such cases one gen- 

 erally finds a tapering stem, usually looped at the end and unprovided 

 with roots or a terminal plate for attachment. A column provided 



