NO. 1846. ON CERTAIN ELEUTHEROZOIC PELMATOZOA—KIRK. 25 



Each of these appendages, I beheve, may be looked upon as a 

 modified marginal plate. The spine has no communication with the 

 interior and the musculature which controls its movements is external. 

 The muscles were doubtless lodged in the lateral, somewhat depressed 

 areas on either side of the terminal opening. The spmes, as I shall 

 subsequently point out, almost certainly had no other function than 

 to raise the adoral portion of the body. It seems possible, further, 

 that they may have functioned to some extent as ambulatory organs. 



The structure of the column is but imperfectly known. It appa- 

 rently is not essentially different from that possessed by other mem- 

 bers of the group, however. It is composed of two longitudinal 

 series of ossicles. In the proximal portion of the column there seems 

 to be a semifusion of the apposed half-segments, resulting in the 

 formation of complete rings. That this fusion has been but imper- 

 fectly acquired is shown by an individual in which the column is 

 broken. In this case the line of fracture clearly indicates the original 

 "longitudinal division of the stem. The column in its proximal por- 

 tion is of large size, but apparently tapers rapidly distad. The stem 

 ossicles are remarkably thin and tliis gives the column a relatively 

 enormous central perforation. This cavity probably lodged the 

 muscles that controlled the movements of the column. 



The articulation of the column with the theca is of considerable 

 interest. It is to be noted, as shown in figure 11, plate 3, that the 

 cavity within which the proximal portion of the column is lodged is 

 quite deep and socket-like in its conformation. All available evi- 

 dence points to unusually free motion of the column within this 

 cavity, both laterally and vertically. Taken in conjunction with the 

 great flexibility of the column itself, and the evidence of a highly 

 developed musculature, it is evident that the stem was capable of a 

 great range of movement. All these facts point strongly to the 

 conclusion that the column had very special functions, quite dift'erent 

 from those normally assigned to the pelmatozoan stem. 



A brief discussion of the habits of this type as the}^ are believed 

 to have been will illustrate clearly the probable functions of the 

 various parts and the reasons -for the modifications to be noted in 

 the gross structure of the organism. It is believed that the animals 

 propelled themselves along the bottom by means of their column. 

 This was pulled forward, elevating the posterior portion of the theca; 

 pressure was then exerted which drove the animal forward. It is 

 here that we see the probable use to which the anterior appendages 

 were put. With the posterior portion elevated and a forward pres- 

 sure applied it is obvious that the tendency would be to drive the 

 adoral part of the animal into the sediment of the bottom. How- 

 ever, were the anterior appendages to be slightly drawn inward so as 

 to elevate the anterior portion of the theca somewhat above the 



