NO. 1S46. ON CERTAIN ELEVTHEROZOIC PELMATOZOA—KIRK. 5 



character and became so irregularly effective that the return to a 

 purely eleutherozoic habit was a comparatively simple matter. In 

 the case of the Blastoidea and Crinoidea, however, the free forms 

 represent aberrant types which have reverted to this type of exist- 

 ence after a considerable period of perfected fixation. 



Divisions of the Cystidea of Group I. — The eleutherozoic Cystidea 

 may be divided into three ''types" or subgroups for the purpose of 

 convenience : 



1. This type is reserved for certain Cystidea which did not have 

 true columns, but when attached at all, were cemented to the bottom 

 by an outgrowth from the body wall. It seems possible from the 

 evidence at hand that certain of these forms were free-swimming or 

 floating organisms, at least during a portion of their lives. 



2. In this type are placed those Cystidea provided with a pre- 

 hensile column which attached themselves at will, probably not by 

 cementation, but rather by looping the distal portion of the stem 

 about some fixed object. 



There is a group of Crinoidea directly comparable mth these forms. 



3. Tliis type includes those Cystidea which propelled themselves 

 along the bottom, making use of their brachioles and possibly their 

 columns as organs of locomotion. 



CYSTIDEA, TYPE 1. 



. In tliis group are chiefly to be found those primitive, many-plated 

 Cystidea which had not yet evolved a column for purposes of attach- 

 ment. These are the forms that in this regard represent the con- 

 nectmg link between tlie eleutherozoic echinoderm ancestor and the 

 Pelmatozoa. With them fixation was a novelty, and in many of the 

 more primitive types, no doubt, was irregularly acquired in the 

 different lines. In some cases a very rudimentary stem seems to have 

 been present, but the possession of such an organ appears to have been 

 an inchfferent character at best. The stem apparently might be 

 absent or not, within the limits of a single species, and when present 

 probably functioned in the same way as the stem in those genera 

 referred to Group III. 



The genera referred by Bather to the families Aiistocystidse and 

 Echinosphgeridse may all provisionally be placed m tliis group. 

 Permanent attachment by cementation to the bottom undoubtedly 

 occurred in many forms, at least in the adult stages, and as noted 

 above, rudimentary stems are occasionally present. In many cases, 

 however, it seems probable that the period of fixation, if there were 

 su'^h, was of but short duration and irregular occurrence. 



Echinosphsera. — The genus Echinosplixra may be taken as charac- 

 teristic of the group. The conclusions to be drawn from the evidence 

 afforded by this genus may be held to apply with greater or less force 

 to the other forms here referred. EcTiinosphaera aurantium, Plate 1, 



