68 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.41. 



The development of the centro-dorsal m the case of Antedon has 

 been described as follows by Carpenter (1888, p. 11): 



The centro-dorsal is at first a simple ring, in no way different from the other stem 

 joints, but when the basals come to assume a definite shape and the calyx acquires 

 the doubly conical form of the Cystid phase, the centro-dorsal becomes distinctly 

 wider than the annular stem joints below it and takes on a pentagonal shape. The 

 basals rest against the sides of the pentagon, and its angles which fit in between them 

 are therefore radial in position. * * * At this early stage the basals are only in 

 contact with the centro-dorsal by their lower edges, but it soon begins to increase in 

 diameter and extends itself over the bottom of the calyx. * * * It increases at the 

 same time in vertical depth, and the first cirri make their appearance. These are 

 radial in position, and the portion of the centro-dorsal between every two sockets 

 rapidly enlarges, so that it comes to project beneath each basal plate, and the angles 

 of the centro-dorsal thus become interradial instead of radial. This change is very 

 clearly seen in larvae which have only one or two cirri, so that one part of the centro- 

 dorsal shows the primitive radial symmetry and another part the acquired interradial 

 symmetry. 



In this form the proximale, without the addition of other columnals, 

 forms the centro-dorsal. 



The further growth of the centro-dorsal is described on page 12: 



1 merely wish to point out that as soon as the centro-dorsal of the early larva of 

 Comatula takes a definite shape its angles are distinctly radial. * * * But when 

 the cirri appear on the centro-dorsal and the basals begin to be transformed into the 

 rosette, the outline of the centro-dorsal changes. The basals are no longer the principal 

 plates in the calyx, but they undergo metamorphosis into the small rosette, and the 

 centro-dorsal increases rapidlyin size, more so than any other part of the skeleton, "so 

 that it soon comes to pass beyond the circlet of basals and to abut on the proximal edge 

 of the first radials; and instead of stopping here it continues to increase in diameter 

 until it conceals the whole inferior surface of the first radials and sometimes even 

 encroaches somewhat on the second . ' ' 



Carpenter, in these passages, as elsewhere, insists upon the primi- 

 tive radial position of the centro-dorsal, as opposed to the views of 

 Wachsmuth and Sprmger (1879-86, pt. 3, p. 298 (222)). Moreover, 

 he proves his point conclusively by giving two figures of larval 

 Antedon, one of which is reproduced here. (PL 7, fig. 6.) In the 

 very early stages, and before the appearance of the cirri, the centro- 

 dorsal conforms to the pentagonal opening formed by the basals, and 

 hence by necessity the angles must be radial in position. 



In their Monograph of the Camerata (1897, vol. 1, p. 64), Wachs- 

 muth and Springer try to controvert the statements of Carpenter 

 above cited. On Plate 6, figs. 18 and 19, they reproduce two figures 

 taken from W. B. Carpenter. Discussing these figures in the text, 

 they make the following statement : 



The centro-dorsal at the Pentacrinoid stage of the Comatulse, aa may be seen by 

 examining PI. 6, figs. 18 and 19, agrees closely with that of the Apiocrinidse. It ia 

 interradially disposed at the proximal face, and also at the distal face, so that its 

 angles correspond with the angles of the basals as in those dicyclic Crinoids whose 

 infrabasals are hidden by the column, and even in the prefloating stage the centro- 

 dorsal retains its interradial position. 



