420 



PELMAT0Z0A. 



Between the five columns of radial plates, corresponding with the 

 five arms, there may be intercalated certain other smaller plates, 

 which, from their position, are spoken of as " interradials " (fig. 

 292, i). When "interradials" are developed, one of the interradial 

 spaces, corresponding with the anus, is usually wider than the others, 

 and is furnished with an additional series of calcareous pieces, which 

 are termed "anal plates" (fig. 292, a). 



As above mentioned, the first brachial plates rest directly upon 

 the highest row of radials ; and the " arms," therefore, spring from 

 the margins of the calyx, where the dorsal and 

 ventral surfaces join. The arms are formed of a 

 single or double longitudinal row of " brachial 

 ossicles," the plates in the latter case alternating 

 with one another. The brachial plates are in 

 many cases traversed by a single or double canal, 

 which transmits the axial nerve-cord (fig. 287). 

 In most cases, the brachial ossicles are movably 

 articulated with one another, having their op- 

 posed surfaces separated by interarticular sub- 

 stance, and being provided with muscular fasci- 

 culi. In other cases, however, the arm-plates 

 are fixed in immovable junction by "syzygy," 

 and may coalesce with one another. The arms 

 are primarily five in number, and they may re- 

 main simple. Most commonly, however, the 

 arms are more or less branched, and they gener- 

 ally carry on their sides short, jointed filaments 

 or " pinnules," the structure of which repeats 

 that of the arms on a smaller scale (fig. 293). 

 Pinnulae may be wanting (as in Cyathocrinus), and it is not always easy 

 to distinguish between " pinnules," properly so called, and " armlets" 

 {i.e., short divisions of the arms themselves) ; since the real distinc- 

 tion between these structures depends upon their contained soft parts, 

 and is therefore unavailable as regards fossil forms. The proper 

 " arms," namely, lodge the sterile genital cord, while it is within 

 the pinnules that the fertile portions of the genital glands are con- 

 tained. Owing to the position of the reproductive glands beneath 

 the soft skin of the pinnules, it follows that there exists no gener- 

 ative opening, or " ovarian aperture," in the walls of the calyx, such 

 as is present in the Cystideans. 



The ventral surfaces of the arms and pinnules, as in Comatula, 

 are furnished with furrows — the " ambulacral grooves " or " food- 

 situated in the direction of the rays, as far as the first axillary joint ; and when 

 the arms are numerous, some authors speak of secondary and tertiary radials 

 according to the number of axillaries between the basals and the free arms. 



Fig. 293. — Portion of 

 an arm of Platycrinus, 

 showing the lateral pin- 

 nulae. 



