4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 65 



THE APPARENTLY NEW STRUCTURES IN THE LATER.CRINOIDS 



In the process of development and specialization of the crinoid 

 phylogenetic line no new features have been added ; nothing is found 

 in the later and more specialized types that does not occur, usually 

 in a more extended form, in the earlier and more generalized. 



There are two apparent exceptions to this statement. The penta- 

 crinites and the comatulids are chiefly remarkable for the great 

 development of cirri, which are unknown in most of the earlier types 

 and which therefore might be assumed to be of relatively recent 

 phylogenetic origin ; and most of the later forms possess one or more 

 series of paired plates, of which the outermost is axillary, inter- 

 polated between the radials and the arm bases, whereas in the more 

 primitive types the arms are given off directly from the radials. 



As is explained further on, in the Articulata the column, after 

 reaching a certain definite length, abruptly ceases further develop- 

 ment, and the last formed columnal becomes permanently attached 

 to the calyx. Though the skeletal development of the column ceases 

 abruptly, the growth of the other constituents of the column is not 

 so suddenly arrested, for we notice that the columnal which is 

 attached to the calyx increases in size and gradually becomes more 

 or less differentiated from the other columnals. If the column be 

 very short — in other words if the suppression of the columnar devel- 

 opment has been very abrupt — cirri are developed which break 

 through the walls of the enlarged topmost columnal. These cirri, 

 invariably associated with atrophied, dwarfed, or attenuated columns, 

 represent a diffuse lateral diversion of the normally linear longi- 

 tudinal stem development. The sudden suppression of the develop- 

 ment of the skeleton of the column is not correlated with a corre- 

 spondingly sudden suppression in the development of the other sys- 

 tems which enter into the columnar structure ; and the organic adjust- 

 ment or equilibrium necessitated by the continued development of the 

 organic portions of the column after the inorganic portion has reached 

 its limit is attained by a lateral diversion of this ontogenetic force, 

 resulting in the formation of a varying number of cirri, each of the 

 cirri representing a fractional degenerate derivative from a sup- 

 pressed column of the normal type, while all of the cirri collectively 

 represent the degree of excess of development possessed by the " soft 

 parts " of the column over that possessed by the skeleton. 



In order to understand the significance of the pair of ossicles in 

 the later ten-armed types which occur between the radials and the 

 arm bases it is necessary to bear in mind that the radials are not true 



