NO. 1846. OA'^ CERTAIN ELEUTHEROZOIC PELMATOZOA—KIRK. 121 



with cirri throughout tho greater part of its length, especiall}^ if these 

 be equipi)ed with terminal claws, such as is sho\\ai on Plate 4, figure 

 2, is an equally good indication that the animal is free-swimming for 

 a greater or less part of its existence. Among >the prostrate Cystidea 

 and Blastoidea, which crawled along the bottom, an eleutherozoic 

 form of existence is clearly mdicated by the marked modifications to 

 be observed in the structure of the theca. These modifications have 

 been discussed at length under such genera as Eleutherocrinus, Pleuro- 

 cystis, and Anomalocystis. 



Aside from such obvious signs of the maintenance of an eleuthero- 

 zoic habit there are many more, the significance of which may only 

 be appreciated when considered in the light of structures brought 

 about tlirough such a type of life. Inasmuch as semi-detached con- 

 ditions so largely obtain among the Pelmatozoa, it is evident that 

 throughout the Grade we have to deal with these modifications. It 

 is only when organs become highly specialized and obviously adapted 

 to eleutherozoic conditions that we feel safe in assigning such an 

 origm to them, however. 



The sporadic deviations and the earlier variations from the stato- 

 zoic stock have characters more or less peculiar to themselves. It 

 should be remembered that these types possessed no long ancestral 

 lines that as Pelmatozoa had assumed a detached existence. As a 

 result, there had come to be no special modifications fitting the 

 forms for such an existence. These earlier types, therefore, are char- 

 acterized by the absence of special organs such as cutI. Instead, it 

 seems that they as a rule had not yet lost the semi-prehensile columns 

 of their ancestors, and could upon occasion attach themselves by 

 coiling the distal portion of tlieu' tapering columns about some 

 stationary object. 



Gradually, as an eleutherozoic existence was more universally 

 assumed, and w^as perpetuated generation after generation, modifi- 

 cations of structure became manifest. Movement probably never 

 was rapid or continuous for any prolonged period of time. As a 

 result, in most forms we may look for but slight modification in the 

 theca of the crinoid. Where modifications became most effective 

 were in the arms and column. 



Effect of a detached mode of life on the column. — The modifications 

 wdiich one would expect to find in the case of the column are of a 

 twofold nature. In the first place the length of the column would 

 be affected, and in the second structures capable of balancing and 

 temporarily holding the animals would be perfected. As is well 

 known, crinoid columns are prevailingly short — at times surprisingly 

 so. To attribute this shortness of column to an eleutherozoic habit 

 would be presumptous, for it is obviously incapable of demonstration. 

 That such a type of life may have been of considerable importance in 



