12 PROCEEDTNGS OF THE NATIONAL MUSEUM. vol.41. 



might be of considerable assistance in determining slight movement. 

 Many of the other forms referred to this group, from the considerable 

 weight of their thecal plates and the generally cumbrous aspect of the 

 body, seem not to be equally well fitted for a pelagic habitat, how- 

 ever. On the other hand, it is impossible to ascribe a sessile form of 

 life to these organisms for the reasons given above. 



If a free swimming existence be barred in such cases we have the 

 alternative of comparatively free motion upon the sea bottom. Owing 

 to the rather feeble nature of the brachial appendages one may not 

 consider them as capable of functioning to any considerable extent 

 as ambulatory appendages. In some cases, as in Protocrinus, where 

 pseudo-ambulacra are well developed there is a possibility of struc- 

 tures comparable in a degree to the podia of the Echinoidea having 

 been present. Even in the case of the Diploporita where numerous 

 vertical canals perforate the thecal walls, it is perhaps not inconceiv- 

 able that organs of a similar function may have been present. The 

 diplopores themselves in certain cases may have harbored such 

 specialized organs. This hypothesis is not as improbable as it may 

 sound at first. We know that in the earliest echinoids such as Both- 

 riocidaris, podia or organs of similar function must have been present. 

 Such a form we may only conceive to have been derived from a line 

 of these many-plated cystids. The chances are, therefore, that podia 

 should be represented in these ancestral forms by similar or equiva- 

 lent organs. Certain it is that we must postulate organs of locomotion 

 of some sort in the very early common eleutherozoic ancestor of the 

 Echinoderma. Lacking brachial appendages what could be more 

 natural than that some such sort of ambulatory organs should have 

 been present ? Upon the assumption of a purely statozoic existence 

 by certain Imes of this primitive stock, such organs would in time 

 become eliminated through disuse. 



There is an interesting structural feature which is apparently inti- 

 mately associated with the assumption of a statozoic existence by 

 these many-plated cystids. This is the presence of a definite central 

 apical plate. It is needless to more than mention the lengthy dis- 

 cussion that has been carried on in regard to the origin and homol- 

 ogies of such a plate. Its significance in this particular group of 

 organisms alone will be discussed at this time. 



It is to be noted in the case of all figures of the bases of these cystids 

 where the sutures of the plates are visible, that there is a well-defined 

 apical plate surrounded by a ring composed of a variable number of 

 plates. This apical plate seems in all cases to be the center of the 

 area of attachment. One may infer that attachment was had pri- 

 marily by means of this plate alone. Whether this plate represents a 

 definite skeletal element of any morphological significance is a ques- 

 tion difficult of solution. I believe it does not. 



