340 BULLETIN 82, UNITED STATES NATIONAL MUSEUM, 



This heavy phitinj:; <>f the disk in the very young of species of which the adults 

 have naked disks must be t)f very profound significance and, when wo consider it 

 in connection with the occurrence of the radianal and of anal x, we are naturally led 

 to the conclusion that it rejiresents a structure once of the highest importance in 

 the economy of the animal, but long since obsolete. 



It is probably to be interpreted as the transient remnant of a solid calcareous 

 ])lating of the same type as that from which the solid vault of the Camorata was 

 developed. 



Orah. 



The orals, though present so far as known in the young of all the recent comat- 

 ulids — indeed in the early stages appearing simultaneously with the basals and of 

 equal importance — are always resorbed long before adult hfe is reached, no trace 

 of them whatever remaining. 



The five orals are always of equal size (figs. 407-413, p. 317, 529,530, pi. 2,532, 

 533, pi. 3, 542-544, 547, 548, pi. 4, 559-564, pi. 6, and 576, pi. 9), no matter how 

 different the sizes of the several interradial areas may later become. Each is an 

 approximately triangular plate, Ij'ing Nvith the apex of least divergence at the 

 peristome; the edge opposite tins apex is more or less convex. In the oligo- 

 phreate species the orals appear commonly to be either a plane triangle, or a spher- 

 ical triangle of lai-ge radius (figs. 408, 411, 412, p. 317, and 548, pi. 4); but in the 

 macrophreate species, as first noticed by W. B. Carpenter, they are neither a plane 

 nor a spherical triangle, for the two edges along which each oral abuts upon its 

 neighboi-s are more or less everted and turned vertically, so that when the orals 

 are closed down they are in lateral apposition with the adjacent orals not by 

 their edges alone, but by the outer side of this everted rim (figs. 409, 410, p. 317. 

 535, pi. 3, 544, pi. 4, and 5.59, 561, 563, 564, pi. 6). This rim is highest at the 

 moutli, where the oral suddenly turns upward, and graduall}- diminishes in height 

 toward the periphery of the disk. 



The orals make their appearance at the same time as the basals (with which 

 among the comatuhds tiicy are strictly correlated in development and metamor- 

 phosis, though morphologically they have nothing whatever to do with them) and 

 long before the radials are formed. Each oral is situated exactly over its corre- 

 sponding basal. 



W. B. Carpenter observed that in Antedon bifida the resorption of the orals, 

 which commences before the termination of pentacrinoid hfe, is completed ver}' 

 soon after the animal has entered upon its free existence. The resorption takes 

 place from the outer edge inward toward the center, the last traces of these plates 

 that can be distinguished being ghstening fragments of calcareous network at the 

 bases of the five membranous valves which still fold over the tentacles forming the 

 oral ring in specimens which have attained a diameter of about an inch and a half, 

 wliich soon disappear entirely. 



As the orals among the comatuhds are essentially a larval structure, further 

 discussion of them is postponed to the section dealing with the Pentacrinoid young. 



In the adults of certain species in which the disk is heavily plated, as in the 



