THE VOYAGE OF H.M.S. CHALLENGER. 



II.— THE CENTEO-DORSAL AND CALYX. 



The principal morphological character which distinguishes the Comatulidse from the 

 remaining families of Crinoids is the development of cirri upon the top stem-joint, and its 

 separation from the remaining portion of the stem as the centro-dorsal plate. This 

 supports the ring of united radials, and, in the recent forms at any rate, closes up below 

 the dorsal extension of the body-cavity which is contained in their central funnel, as is 

 well shown in PI. III. figs. 3a, 3b, and PI. V. fig. 2c. 



Most recent Comatulse are further distinguished from the Stalked Crinoids by the 

 metamorphosis of the embryonic basals into the structure known as the " rosette," which 

 is enclosed within the radial pentagon, and so is entirely invisible externally (PL I. 

 fig. 8c; PL II. figs. 3c, 5c; PL IV. fig. 3c; PL V. figs. 2c, bd). It will be well 

 to discuss these two structures separately, though they are naturally in very close relation 

 with one another. 



A. The Centro-Dorsal. 



The term " centro-dorsal plate " is a very old one, and was for a long time used in 

 various ways by different authors. In fact it was not till the remarkable developmental 

 history of the uppermost stem-joint had been made out by the late Sir Wyville Thomson 

 and Dr. Carpenter, that the term acquired any definite signification. Both these authors 

 used it to denote the enlarged and cirrus-bearing top stem-joint 1 which is at first in no 

 way different from the remaining joints of the stem below it (PL XIV. figs. 1, 2, 8, 9). 

 Eventually, however, it enlarges, and five cirri, which are radially situated, are developed 

 upon it (PL XIV. figs. 3-6), so that it has very much the appearance of a nodal stem- 

 joint of Pentacrinus. A second series of cirri, alternating in position with the first, 

 subsequently appears (PL XIV. fig. 7), and others are afterwards developed in succession, 

 so that as was well said by Wyville Thomson, 2 " the centro-dorsal plate in Antedon does 

 not belong to the cup. It represents a coalesced series of the nodal stem-joints in the 

 Stalked Crinoids." 



At a certain period in the development of the young Comatula the centro-dorsal 



1 The centro-dorsal plate of Comatula must not be confused with the dorsocentral plate of other Echinoderms. 

 This name is now generally restricted to the central plate of the abactinal system in Urchins and Stellerids. I believe 

 this to be represented in the Comatula? by the terminal plate at the bottom of the larval stem, as explained on p. 168 of 

 Part I. It is shown in PI. XIV. figs. 1, 9. Comatul* thus have both a centro-dorsal and a dorsocentral, while the 

 latter only is present in the remaining Echinoderms. Zittel has also given the name centro-dorsal to the enlarged 

 uppermost stem-joint of Apiocrinus; but this bears no cirri, and though undoubtedly homologous with the centro-dorsal 

 of Comatula;, should not, I think, receive a name which is now universally understood as denoting the presence of 

 cirri. 



2 On the Embryogeny of Antedon rosaceus (Linck, Comatula rosacea of Lamarck), Phil. Trans., 1865, p. 536. 



