NO. 1846. ON CERTAIN ELEUTHEROZOIC PELMATOZOA—KIRK. 15 



very large proportion of the stalked Cystidea not included in Type 

 3 or not otherwise disposed of in the present group may be held as 

 erect, detached forms. Among the more primitive Cystidea, particu- 

 larly, it seems highly probable that permanent fixation was of com- 

 parativeh'' rare occurrence. It is among these more simple types 

 particularly that we are able conclusively to prove nonfixation, for 

 here we have complete columns more frequently preserved. Among 

 the later and more specialized cystids the structure of the column 

 and its marked flexibility, wliicli is frequently made more apparent 

 through a looping of its distal portion, indicates anytliing but adap- 

 tation to a purely statozoic form of life. Wlierever the type of 

 column heretofore described as generally typical of the greater num- 

 ber of forms referable to this group is found, I think we may be safe 

 in assuming an eleutherozoic habit. 



Arachnocystis. — The most primitive type that may be assigned to 

 this subgroup is Arachnocystis, a genus with the highly interesting 

 EchinospJiserites infaustus of Barrande as the genotype. In this 

 form we have one of the most rudimentary colunms laiowTi among 

 the Pelmatozoa. In other genera we have evagination of the body 

 wall and marked constriction of the basal portion of the theca, but 

 here is to be found the simplest column that may be called such with 

 justice. In Arachnocystis we find that the column has been differ- 

 entiated to the extent of being composed of a regular vertical series 

 of imbricating plates therein differing from a mere evagination of the 

 thecal wall. Figures 5, 6, Plate 2, give a fair notion of the structure 

 of the column. From the figures given, there appear to be five rows 

 of plates, but according to Barrande (1887, p. 28) there may be five 

 or six ranges. The column is in most cases of marked brevity, none 

 figured exceeding a length of 50 mm. There is in no case where the 

 distal extremity is preserved any indication of attachment by the 

 organism. 



The column of Arachnocystis mast have differed functionally as 

 well as structurally from that of the more specialized members of this 

 group. Although mobile to a certain extent, the column of Arach- 

 nocystis was not sufficiently slender nor properly constructed to 

 function as a prehensile organ. Subsequent to the assumption of an 

 eleutherozoic habit, which obtained during the adult stages of the 

 animals, such a column could not have been other than a hindrance. 

 The sole service it could render the cystid would be as a sort of ballast. 

 Considering the relative insignificance of the column compared with 

 the bulk of the theca, this function must have been of inconsiderable 

 consequence. 



It is evident that this genus has been derived from a type not 

 greatly dissimilar to Echinosphsera. The slight protuberance to be 

 noted in that genus has, however, been considerably accentuated. 



