DISCOVERY OF THE DISK OF ONYCHOCRINUS 473 



crinoid stage, between the time of emergence from the embryo and 

 that of casting off the stem, reference being had to the figures herein : 



The animal consists in the first instance of the stem and calyx 

 alone, not even rudiments of arms being distinguishable. The calyx 

 completely incloses the visceral mass, its oral valves when drawn 

 together meeting over the mouth, and when these open out it takes 

 the form of an inverted bell. Its oral part is composed of five oral 

 plates, the approximation of which forms a five-sided pyramid with 

 its truncated apex pointing upwards — though they are usually erected 

 as separate valves, to allow the oral apparatus to be projected from 

 within them. The mouth of the bell, when the oral valves are ex- 

 panded, is entirely occupied by the oral tentacular apparatus. The 

 general aspect of the young Pentacrinoid, as seen with the tentacular 

 apparatus retracted in a spirit specimen, is shoAvn in Plate V, Fig. i. 

 The oral plates are triangular in form, their apices pointing upward, 

 but capable of closing so as to form a five-sided pyramidal cover to 

 the calyx (Plate V, Fig, 3). With the further development of the 

 Pentacrinoid, a remarkable change takes place in the relative posi- 

 tion of these orals. They do not increase with the other elements 

 of the growing calyx. They continue to embrace the circle of oral 

 tentacles, the diameter of which comes to bear a smaller and yet 

 smaller proportion to that of the ventral surface of the disk; and 

 thus it comes to pass that the circlet of oral plates detaches itself 

 from the summits of the radials on which it was previously super- 

 imposed, and is relatively carried inward by the great enlargement 

 of the circle formed by the latter — the space between the two series 

 being now filled in only by the membranous perisome, which is 

 traversed by the five radial canals (ambulacra) that pass out from 

 the oral ring between the oral valves to the bifurcation of the arms 

 (Plate V, Fig. 7). 



Before the body of the Pentacrinoid drops off its stem, an incip- 

 ient resorption of the oral plates is discernible, commencing along 

 the margins of the apical portion, so that these plates lose their tri- 

 angular form and become somewhat spear-shaped (Plate V, Fig. 7). 

 This absorption continues in various degrees; sometimes the upper 

 half of the oral plate disappears, and in other cases the marginal 

 portiorls only of the upper part of the plate have been removed, 



