72 STUDIES IN THE DEVELOPMENT OF CRINOIDS. 



10. THE SKELETON. 



The order of appearance of the primary skeletal plates in the Crinoids 

 studied here is in accordance with what is known from Antedon, the oral 

 and basal plates, together with the terminal stem-plates, being the first to 

 appear, the two sets of calycinal plates lying originally in two half-circles 

 open ventrally, not exactly opposite one another, and assuming their final 

 position only about the time of the metamorphosis. The plates of each set 

 do not develop quite contemporaneously, as is seen in figures 1 and 2 of plate 

 ix and figure 3 of plate xn, but it was not found possible to ascertain their 

 exact order of appearance. After the orals and basals follow the radialia, the 

 costals, and axillaries; a remarkable exception to this general rule is afforded 

 by Florometra serralissima, in which the axillaries appear before the radials, 

 the costals being the last to develop (plate xxvn, figure 6). 



According to Perrier (op. tit., pages 219, 220), the arms do not develop 

 contemporaneously, but successively, in a definite order, as illustrated by his 

 figure 18 of plate 2. The observations recorded in the present memoir on 

 Compsometra, Isometra, and Thaumatometra, do not lend the slightest 

 support to this statement and the same holds good for the observations 

 previously recorded on the Pentacrinoids of Hathromelra prolixa, K as well 

 as the observations of M. Sars on Antedon (Hathrometra) sarsi M and of 

 Wyville Thomson and W. B. Carpenter on Antedon bifida. Also the figure 

 of a Pentacrinoid of Comactinia meridionalis, figured by A. H. Clark, in his 

 "Monograph of the Existing Crinoids" (page 317), shows the young arms 

 equally developed; and the same is the case in the Pentacrinoids figured 

 by P. H. Carpenter in his Challenger Comatulids, plate xiv. There can 

 then be no doubt that it is a general rule in Comatulids that the arms develop 

 contemporaneously. Even in Florometra, where the radials, costals, and 

 axillaries are developed successively, not at the same time in all the radii, 

 the arms are all of the same size in the young Pentacrinoids (plate xxvn, 

 figure 5). The Pentacrinoid represented by Perrier in the figure quoted 

 is evidently abnormal. 



Regarding the oralia, it should be mentioned that the observations 

 recorded in the present memoir are in accordance with the statement of 

 A. H. Clark (Monograph of the Existing Crinoids, page 340), that in the 

 macrophreate forms they are concave with out-turned edges, while in the 

 oligophreate they are flat or convex, the edges not out-turned. This latter 

 form they have in the oligophreate Tropiometra, while in Compsometra, 

 Isometra, Florometra, and Thaumatometra, all macrophreate (of the family 

 Antedonidce) , they have the markedly out-turned edges. This difference 

 between the Pentacrinoids of the two main groups of Comatulids thus really 



68 Th. Mortcnscn. Report on the Echinoderms collected by the Danmark-Expedition at Northeast 

 Greenland, Medd. om Gr0nland, XLV, 1910. 



69 M. Sara. Memoircs pour servir a la connaissance des Crinoides vivants, pi. v, 1868. 



