206 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



attached by this soft integument which then lengthened out into a slender stalk, 

 resembling the stalk of such forms as Boltenia, carrying with it, of course, the spi- 

 cular investment, the included calcareous deposits of which increased in number 

 and became segregated into definite ossicles. Such an origin for the column would 

 accord with what we know of the origin of the cirri and of the distal portion of 

 the pinnules. 



This would make it clear at once how it is that the prolongations from the 

 chambered organ and the associated structures became continued into the column; 

 but while there is evidence that something of the kind may have occurred in certain 

 of the older fossils, it does not seem to have occurred in any of the recent types 

 nor in any of their immediate fossil representatives. 



The elongation of the apical plate as presupposed in the two first alternatives 

 does not necessarily call for a uniform deposit of stereom all over its internal surface. 

 The chambered organ and the accessory structures probably retained their original 

 relationship with its center and became drawn out into a complex axial cord as a 

 result of the deposit of stereom about the periphery; or the new columnals, formed 

 just within the apical plate, arose as rings (as the topmost columnals do in ah" of 

 the recent forms) which grew inward until the distal portion of the elongated 

 chambered organ was reduced to a very small diameter. 



As described above, these three possible origins of the column and of the indi- 

 vidual columnals would appear to be very different, but upon consideration it be- 

 comes evident that the difference is more in words than in fact. We are probably 

 nearest the truth if we consider that all three alternatives play a part in the for- 

 mation of the crinoid column, but place the greatest emphasis upon the second. 



The columns of the later and recent crinoids in general differ from those of the 

 earlier forms in developing with much greater rapidity, though this is masked by 

 the fact that they possess also a definite growth limit at the attainment of which 

 further development ceases, such a growth limit being unknown in the palaeozoic 

 types. 



A series of loose sutures is mechanically available only for slowly growing 

 columns, in which the individual columnals are very short. With increasing pro- 

 portionate length the loose sutures between the columnals gradually undergo a 

 differentiation; a fulcral ridge develops, and the ligament fibers become segregated 

 into two large bundles, one on either side of it. 



It is by this process that a column formed according to the second hypothesis 

 becomes transformed into the type characteristic of the later fossil and the recent 

 crinoids. 



There is a definite limit to the possibilities of further growth in a column com- 

 posed of long ossicles fastened end to end by alternating articulations consisting 

 of two ligament masses separated by a fulcral ridge. If the animal remains small 

 with a small light crown, such a column may safely attain a length of 100 or more 

 columnals, but if the crown should become of large size and heavy, a stem of this 

 type would not be able to support it; the rapidly increasing tendency to "buckle" 

 would limit the available length of a stem of this nature. 



