NO. 1846. ON CERTAIN ELEUTHEROZOIC PELMATOZOA—KIRK. 21 



the column are, in all cases that have come under my observation, 

 somewhat coiled. In two or three specimens that I have examined 

 there could scarcely have been more than half an inch of the distal 

 portion of the columns lacking. Possibly the stems were complete. 

 In no case was there the slightest trace of rootlets, and no evidence of 

 a distal basal expansion such as is common among the Crinoidea. 

 The evidence seems to point to the conclusion that Pleurocystis was 

 never firmly rooted to any one spot but anchored itself by hooking 

 the tenuous distal portion of its stem about some extraneous object. 



The analysis of the theca and the figures of Pleurocystis as given by 

 Jaekel and Bather are inaccurate in several important features. 

 Plates 1 and 4 as given by them should be split vertically and por- 

 tions united with 2 and 3. Plate 13 is not present in the cup, the 

 apophyses of Plates 12 and 14 uniting without the intervention of 

 another plate. This plate vras present in very primitive types but 

 was crowded out subsequently. A full discussion of this subject will 

 be given in a paper which treats in detail of the Pleurocystidse. In the 

 present paper it was thought best rather to illustrate an actual speci- 

 men than to give a reconstruction. It will be noted that the structure 

 of the column is quite different from that given by either Bather or 

 Jaekel. 



The marked asymmetry of Pleurocystis is unquestionably secondary. 

 What the ancestral erect cystid was like is an interestmg question. 

 Bather, in Lankester, derives Pleurocystis from Cheirocrinus and 

 places both genera in the subfamily Glyptocystidse. I am personally 

 inclined to derive Pleurocystis from a form having essentially the 

 structure of Echinoencrinus. This genus is not the ancestor of Pleuro- 

 cystis, but both were probably derived from a common ancestor not 

 far removed. 



CYSTIDEA, TYPE 3. 



We have here probably the most aberrant type of evolution to be 

 found among the Echinoderma, and one w^iicli in the nature of tilings, 

 one would least expect. To fuid among the Pelmatozoa a group of 

 organisms that are perhaps more highly specialized for purposes of 

 locomotion than a large proportion of the Eleutherozoa is a novel 

 experience. So milike Echinoderms are they in certam respects that 

 some of them have been described as Crustacea, which indeed they 

 closely simulate. The members of this group moved actively over 

 the bottom, using both column and "brachioles," when present, as 

 organs of locomotion. In this regard, they may be looked upon as 

 going a step beyond that reached by type 2, and are closely com- 

 parable to EleutJierocrinus. The departure from the beaten path 

 seems to have been quite successful, for these types range from the 

 Cambrian to the very latest cystid appearance in the Devonian 

 (Oriskany). 



