348 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



nnd cdmcs to possess a second (anal) orifice. The orio;inal basals have undergone 

 htlh>, if any, increase, but the radials are now much larger and s|)read out so as to 

 extend to the base of the cup instead of forming its sides. This spreading out 

 results from the increase in their own breadth without a corresponding increase 

 in the diameter of tlio circle on which they rest, so that they are forced to extend 

 themselves obhquely instead of vertically. The anal plate, being attached not so 

 much to the adjacent i)lates as to the visceral mass, begins to be lifted out from 

 between them with the development of the anal funnel, and the space left by it is 

 partly filled up by the lateral extension of the two radials between which it was 

 previously interposed, bxit wliich do not as yet come into mutual contact. The 

 primibrachs also increase in all their dimensions, but particularly in breadth, and 

 they thus assist in supporting the visceral mass which, at the conclusion of this 

 stage, extends itself as far as the bifurcation of the arms. The most remarkable 

 change in the condition of the calcareous skeleton in this stage, however, con- 

 sists in the altered relative position of the orals; these do not partake of the enlarge- 

 ment so remarkably seen in the radials, nor do they become more separated from 

 each other. The cu-clet of orals continues to embrace the circle of oral tentacles the 

 diameter of which comes to bear a smaller and smaller proportion to that of the ven- 

 tral surface of the disk, as the size of the latter is augmented b}^ the development of 

 tlie intestiniil tube around the gastric cavity, and thus it comes to pass that the 

 circlet of oral plates detaches itself from the summits of the radials on wliich it was 

 previously superimposed, and is relatively carried inwaid by the great enlarge- 

 ment of the circle formetl by the latter, the space between the two series being 

 now filled in only by the membranous perisome which is traverseil by the five 

 radial canals that pass out from the oral rmg between the oral valves to the bifura- 

 cation of the arms. During the latei stages of pentacrinoid life the calyx is still 

 more opened out by the increased lateral as well as longitudinal development of 

 the radials, but the diameter of the disk augments in even larger ])roportion, so that 

 it extends nearly as far as the bifurcation of the arms. The oral circlet is thus sepa- 

 rated by a much wider interval from the periphery of the disk, and in this outer 

 ring the anal funnel is now a very conspicuous object, the anal ])Iate which it bears 

 on its outer side being altogether lifted out from between the two radials which 

 it originally separated. Before the body of the pentacrinoid drops off its stem 

 an incipient resorption of the orals is discernable; this resorption commencing 

 along the margins of the ajjical portion so that these plates lose their triangular 

 form and become somewhat spear shaped." 



In the comatulids the radials compose the only circlet of body plates per- 

 sisting as such to the adult stage (except in the genera Atelecrlnus and Aiopocrinus, 

 where there is also a circlet of urmietamorphosed basals), the infrabasals (when 

 present at aU) having early become united with the centrodorsal, and the basals 

 at a later stage having moved inward and become completely metamorphosed into 

 the rosette, or possibly in some cases entirely resorbed (figs. 6G-6S, p. 93, and 431, 

 432, p. 349). 



