30 STUDIES IN THE DEVELOPMENT OF CRINOIDS. 



apparently remain quite short, as was also found by M. Sars in Hathromelra 

 sarsii. Also in Antedon bifida a few of the upper stalk-joints are somewhat 

 wider than the rest of the stalk-joints, as is shown distinctly in plate xxxix, 

 figure 2, of W. B. Carpenter's memoir; but it is not so prominent a feature 

 here as in Hathrometra and Compsometra, and less constant ; in some specimens 

 there is hardly any widening at all discernible in the upper joints. 



The shape of the fully formed joints appears from figure 5, plate xm. 

 While in the joints of the young Pentacrinoids the middle plate is wider than 

 the rest of the joint, and thus appears as a prominent ring around the middle 

 of the joint, the fully formed joints are a little narrowed in the middle, 

 slightly hourglass-shaped, and the middle plate appears only as a more or 

 less distinct line across the middle of the joint. The ends of the joint are a 

 little widened. The articulating surfaces are alternating, as usual in Penta- 

 crinoids, whereby the impression is produced that the joints are united two 

 and two. Sometimes, however, the articular surfaces at both ends of a joint 

 may be identical, not alternating; this is the case, for instance, with the upper 

 joint in the small piece of the stalk shown separately in plate xin, figure 5. 

 The joints are smooth and short, the longer of them only about 0.25 mm. 

 The terminal stalk-plate is small, slightly lobed. 



The young Crinoid is detached very soon after the stage figured in plate 

 xm, figure 5. I have found a detached specimen with only two pinnules 

 developed and a third beginning. There are only 8 cirri, viz, the 5 of the 

 primary whorl and 3 of the second, one about as long as the primary ones, 

 the second half as long, and the third quite small; the second is in the anal 

 interradius, the first to the left, the third to the right of it. The oral plates 

 and the anal plate are still distinct, but the absorption has begun. That the 

 specimen has just been detached is evident also from the fact that a small 

 hole is still seen in the middle of the centrodorsal. 



In another young specimen, the above-mentioned one with the oral 

 pinnules about to develop, the anal and oral plates have entirely disappeared 

 and the skin of the disk is quite naked, as is also the case in the grown 

 specimens. 



A Pentacrinoid of the Australian Compsometra loveni (Bell) was figured 

 by A. H. Clark in his monograph of the existing crinoids (Bull. U. S. Nat. 

 Mus. 81, 1915, p. 317, figure 410). It appears to be a stage where the radials 

 have just begun to form, slightly younger than the stage of C. serrata figured 

 in plate xm, figure 2. The difference between these two Pentacrinoids would 

 at the first sight appear to be very great still this is probably mainly due to 

 the different way in which the figures have been drawn, that of C. loveni 

 being apparently somewhat diagrammatic. It appears, however, that in 

 this species also the basalia embrace the lower end of the oralia. The latter 

 would seem to be more rounded than is the case in C. serrata. 



