MOKOGRAPH OF THE EXISTING CRICOIDS. 217 



calcareous segments in the column appears to cease, and a remarkable change 

 begins to show itself in the one on wliich the calyx rests. Instead of increasing in 

 length, its original annular disk augments in diameter, becoming convex on its 

 lower surface and concave on its upper, and it extends itself over the bottom of 

 the cal)^ in such a manner as to receive into its concavity the apices of the basals. 

 This change commences while the calcareous segments next below are still rudi- 

 mentary, so that although no further increase in the number of segments takes 

 place subsequently, yet some increase in its length will still be effected by the com- 

 pletion of the last formed columnals, previously immature. The total number 

 of columnals in a pentacrinoid column is subject to considerable variation, ranging 

 (in Antedon lifida) from 16 to 24, the average being about 20." 



"Soon after the highest segment of the column begins to enlarge we notice 

 on that portion of its under surface that extends beyond the columnal upon which 

 it rests one or more minute tubercles which arc the origins of the dorsal cirri. 

 Each of these tubercles is formed bj- a projection of the sarcodic substance of the 

 perisome, within which are observable one or more minute annular disks of calca- 

 reous reticulation. The projection of the tubercle gradually increases, and the 

 number of disks (which are the rudimentary cirrals) is multiplied, so that each 

 incipient cirrus presents the form of a short cyhnder, marked by transverse annu- 

 lations. The length of this cylinder is progressively augmented by the formation 

 of new disks and by an increase in the thickness of the earUer ones, and the ter- 

 minal segment soon presents an indication of the peculiar character it is ultimately 

 to assume. As each cirrus elongates, its extremity, at first bluntly rounded, 

 becomes pointed, the terminal segment developing itself into a conical form, though 

 still covered with the same investment of condensed sarcode as extends over the 

 entire length of the rudimentary cirrus. The cirri of the first whorl alternate in 

 position with the radials; they are not developed at the same time, but progres- 

 sively about the periphery of the centrodorsal, the first one, corresponding in posi- 

 tion to the commencement of the intestine, usually exhibiting numerous segments 

 and a conical termination before the fifth, which is opposite the radianul plate, 

 appears. 



" In the later stages of pentacrinoid life the column shows no increase in the 

 number of its segments, but those last formed are developed to almost the same 

 length as the rest, and all the columnals are somewhat augmented in diameter 

 toward their extremities so as to present somewhat of the 'dice-box' form. The 

 original annulus, which is still distinguishable in the middle of their length, so far 

 from constituting a projection, now lies in a hoUow. The axial cavity, if not quite 

 obliterated by the fdling up of the segments, is very much contracted; on tliis pobit 

 it is difTicult to arrive at a positive determination. The comiection of the columnals 

 by a distinct fibrous tissue resembhng that of the arms, and not merely pa.ssing 

 from one articular extremity to the other, but also embracing the contiguous 

 extremities which it connects, now becomes obvious. 



"The most important change which the column presents at this period con- 

 sists in the enlargement of its highest basin-shaped segment, from which the dorsal 



-OMC— Bull. 82—15 15 



