MONOGRAPH OF THE EXISTING CRINOIDS. 219 



mcluded in it. Numerous cases of such elongatidU of a part of the body wall are 

 found among the echinodorms as well as in many other groups. 



It has been suggested that the columns of crinoids originated thus from the 

 prolongation of the posterior part of the body of a more or less irregularly plated 

 primitive ancestor, the plates carried out into the primitive column becoming later 

 reguharly arranged. Aside from the objection that I can not imagine the ancestral 

 crmoid ever to have possessed an irregularly plated apical portion of the body, I can 

 see no reason for supposing that the columns of the recent crinoids and of their 

 immediate fossil representatives were derived through any such process. I consider 

 that the tj'pe of column which is composed of so-called ])entameres represents a 

 different sort of structure entirely from that seen in the recent crinoids, a develop- 

 ment from a spiculated apical area instead of from a definite central plate, though 

 the perfected form of both is identical. 



Centrodorsal. 



The centrodorsal, from which the cirri arise, is the modified topmost columnal 

 of the pentacrinoid larva, and as such is homologous with the so-called proximale, 

 and with the nodals of the pentacrinitcs. 



Being the exact equivalent of the proximale, it represents each nodal of the 

 pentacrinite individually, and, as each nodal is merely a twinned reduplication of a 

 pruuarily single proximale, it also represents all the pentacrinite nodals coUectivoly. 



Sir Wyville Thomson and W. B. Carpenter stated the exact truth when they 

 wrote that the centrodorsal represents a coalesced series of pentacrinite nodals: but 

 unfortunately they failed to appreciate the true homologies and significance of the 

 nodals, and therefore, while their statement was entirely correct, it has invariably 

 been mismterpreted by subsec^uent authors. 



In the later fossil ami in the recent crinoids, as has been explained in the preced- 

 ing pages, the column possesses a definite growth limit upon reaching which aU 

 further development ceases, while the topmost colunmal enlarges and becomes 

 permanently attached to the ajiical portion of the calyx by close suture, and to the 

 columnal next below by a modified close suture or stem syzygy. Thus these crinoids 

 ty]3ically possess a column always with a definite number of columnals, the topmost 

 of which has become to all uitcnts and purposes an apical calyx plate attached to 

 what is now the top of the column by stem syzygy. 



The column of the pentacrinoid larvas just before the formation of the cirri is 

 the characteristic column of the later fossil and recent crinoids develojied in its most 

 tyjiical form. But after the growth limit has been reached the ])roximale continues 

 to develop, gives rise to radiating cirri, and finall}', having become far too large for 

 the slender column to support, breaks away from the columnal just beneath it by 

 fracture at the syzygy between them. 



The numerous cirri on the j>eriphery of the adult centrodorsal very naturally 

 gave rise to the idea that possibly this plato was a composite, the resultant of a 

 process of fusion uniting several individual columnals; but W. B. Carjtenter proved 

 conclusively that in Antedon bifida it is formed hy the eidargemeut of the topmost 

 columnal alone, no others entering into its construction. 



