180 ECHINODERMATA PELMATOZOA SUB-KINGDOM in 



the plates forming the side- walls of the calyx, being absent from its upper 

 surface. In still other instances (Pleurocystites, Callocystites), (Fig. 297), the 

 pore-rhombs are greatly reduced in number, and occur in the form known as 

 "pectinated rhombs." The component halves of the latter stand on contiguous 

 plates the same as the ordinary pore -rhombs, but are always separated 

 externally by an interval ; frequently the two parts are of different form or 

 size, and sometimes one of them may become obsolescent. 



As regards the functions of these canals and pores (the " hydrospires " of 

 Billings), the anatomy of existing Crinoids furnishes us with no positive con- 

 clusions. They have been compared with the pores which are present in the 



tegmen of the . latter, and 

 the rather plausible sugges- 

 tion has been offered that 

 they served to admit water 

 into the body -cavity, and. 

 thus performed respiratory 

 functions. At all events, 

 they could not have served 

 for the protrusion of tube- 

 feet, since they are fre- 

 quently covered over by an 

 FlG - 297 - outer calcareous membrane, 



Callocystites Jewetti, Hall. Silurian (Niagara Group); Lockport, whip}, pffppfnall v <5>inf<? off 



New York. A, Calyx from one side (natural size). B, Ambulacral wm tuaiiy SHI 



grooves and three pectinated rhombs (rh), enlarged ; o, Mouth ; communication with the 

 an, Anus ; g, Genital pore (after Hall). 



exterior. 



The arms in the Cystideans are but feebly developed, and are sometimes 

 entirely (1) wanting, or they are but few in number (2, 3, 6, 9-13). The pent- 

 amerous symmetry, so generally characteristic of Echinoderms, pervades neither 

 the arrangement of the calyx plates nor the number and disposition of the 

 arms. The latter are invariably simple, are either uniserial or biserial, and 

 exhibit a ventral groove protected by covering plates. 



In some genera the arms attain considerable thickness, but in others they 

 are very diminutive, and seem to have closer affinities with pinnulae than with 

 the arms of Crinoids. In the Callocystidae and Agelacrinidae, as well as in the 

 Canadian genera, Amygdalocystites and Malocystites, the arms either recline with 

 their dorsal side facing the calyx, or they are incorporated into the calyx. 

 The ventral side, in these cases, is directed outwardly, and the ambulacral 

 furrow is bordered on either side by a row of alternating, jointed pinnules, 

 which are attached by small articular facets running parallel with the 

 groove. Pinnules have not as yet been observed in Agelacriiws, but the 

 allied genera, Mesites and Asteroblastus, are furnished with delicate facets for 

 their attachment. 



The stem, as a rule, is greatly abbreviated, and is frequently obsolete. 

 Sometimes the calyx is attached by the entire lower 'surface (Agelacrinus) ; or 

 in other cases by means of a tubercular process (Echinosphaerites). Only in 

 rare instances does the stem appear to have served for attachment, since it 

 generally tapers dis tally to a point, and is invariably destitute of cirri. The 

 stem sometimes resembles that of the Crinoids, in being composed of a number 

 of short, prismatic, or cylindrical joints ; these are pierced by a wide canal, 

 and are either united by horizontal, striated, articular surfaces, or they over- 



