A MONOGRAPH OF THE EXISTING CRINOIDS 357 



In young specimens a considerable portion of the radials is visible, but the projec- 

 tion of the edge of the next following ossicle is almost as marked as in the mature 

 individual. In the youngest specimen with the arms about 40 mm. long the external 

 surface of the radials is rather less wide than that of the IB^ and a trifle more than 

 half its length. It does not, however, increase in size with the corresponding parts of 

 the IBr series, but remains undeveloped and is sometimes marked by small tubercular 

 elevations like those on the centrodorsal from which it is with difficulty distinguish- 

 able. These are situated in the gap between the ventral edge of the centrodorsal and 

 the proximal edges of the IBri which project backward so as to overlap them. 



Carpenter said that the most striking feature of acoela, and the one which allies 

 it most closely to the Basicurva group, is the great size both of the segments and of the 

 protecting plates of the genital pinnules. Even the pinnules of the second pair (P 2 

 and P b ) are enlarged for the reception of the gonads, three of their middle segments 

 being expanded; and a little farther from the disk the fifth and the four or five follow- 

 ing segments are flattened and produced laterally, the proximal segment being often 

 much enlarged at the same time. This expansion is not almost entirely limited to 

 the outer side only as in Charitometra incisa, but it is equal on both sides of the medio- 

 dorsal line, and the ventral portion of these expanded segments is covered by an arched 

 pavement of strong plates, few in number but of large size and often very regularly 

 arranged. These protecting plates are much larger and better developed than in 

 either Charitometra incisa or Glyptometra tuberosa, and they often alternate more or 

 less regularly on opposite sides of the medioventral line of the pinnule, where there is 

 an opening in one of them for the exit of the genital products. 



In the young individuals obtained, even in those with an arm length of 60 mm., 

 there is no trace of the enlargement either of the pinnule segments or of the protecting 

 plates, although both are visible in the older form which still shows a considerable 

 part of the radials externally. In the regenerated arm, too, the lower pinnules are for 

 some time quite small and inconspicuous and altogether different from those of the 

 uninjured mature individual. This is the case even when the arm has attained almost 

 its full size and is absolutely larger than those of other individuals not yet quite mature, 

 but with comparatively large genital glands. 



All these greatly enlarged genital pinnules are devoid of ambulacra, like the non- 

 tentaculiferous posterior arms of the Comasteridae; but at about the position of the 

 twenty-fifth brachial there is a sudden diminution in size both of the pinnule segments 

 and of the protecting plates, more especially of the latter. They become much smaller 

 and relatively more numerous, while the sacculi, which are absent from the large lower 

 pinnules begin to appear, just as they show themselves in the genital pinnules of 

 Glyptometra angusticalyx from the same station, while eventually the ambulacra! skele- 

 ton shows itself above the small protecting plates, as in Glyptometra incerta. A little 

 farther out on the arms these protecting plates disappear, and the ambulacral skeleton 

 comes to rest directly upon the pinnule segments. The side plates are very well dif- 

 ferentiated and are often notched for the reception of the sacculi or of portions of 

 them; but in other cases when the sacculi are large they are altogether covered and 

 concealed by the side plates. 



In his memoir on the stalked crinoids of the Challenger Expedition Carpenter 

 noted that on the disk of Poecilometra acoela the double row of transversely oblong 



