NO. 1846. ON CERTAIN ELEVTHEROZOIC PELMATOZOA—KIRK. 13 



Fixation may have become effective so early in the ontogeny of 

 such cystids as to have preceded the formation of a definite skeletal 

 structure. If such were the case attachment would take place at 

 the aboral pole irrespective of plate orientation. Under such cir- 

 cumstances it is obvious that at the point of fixation a plate would be 

 formed, and that the circumjacent plates would of necessity assume 

 a somewhat definite arrangement. Even had fixation occurred sub- 

 sequent to the formation of definite skeletal elements, it is apparent 

 that the point of cementation must have been a plate, and that this 

 plate must necessarily be located somewhere near the aboral pole. 

 Subsequent to the attachment of the animal, a readjustment of the 

 plates of the basal portion of the theca must have taken place, which 

 would give the proximal ring as we find it. The apparently variable 

 location of the point of fixation seems to preclude the possibility of 

 cementation constantly taking place through the agency of a definite 

 plate. 



According to this hypothesis then, the formation of the central 

 apical plate is a secondary matter and entirely dependent upon the 

 fixation of the animal. It may well be that in the phylogeny of the 

 P-elmatozoa this plate might acquire a certain fixed status and be per- 

 petuated as a definite element, perhaps homologous to the terminal 

 ossicle of the pentacrinoid stem of Antedon. 



CYSTIDEA, TYPE 2. 



This group, as opposed to the preceding, consists of Cystidea which 

 have well-developed columns. It is probable that all of the forms 

 here referred were freely moving organisms, during the greater part 

 of their lives at least. Some maintained an erect position, while in 

 the case of other forms we note a tendency toward the assumption 

 of a prostrate habit. This tendency becomes more marked and 

 reaches its maximum development among those forms referred to 

 the next subgroup. The column in all cases probably functioned 

 merely as an organ for temporary attachment. Attachment in tliis 

 group was apparently never by means of radicular cirri, and seldom by 

 cementation. In the majority of cases the distal portion of the 

 column seems to have been more or less prehensile and it is probable 

 that anchorage was effected by looping this portion of the stem about 

 some stationary object. In all these forms the entire stem appears 

 to have been quite flexible. 



Structure of stem in tyj)e 2. — The columns possessed by such Cys- 

 tidea, with the exception of the most primitive types, are essentially 

 similar in structure. The columnals are circular and are of large 

 size next the theca. Distad the diameter of the column decreases 

 rapidly. After a certain point, where the stem is comparatively 

 slender, the column continues to taper, but at a greatly diminished 



