148 



ECHINODERMATA— PELMATOZOA 



PHYLUM IV 



Pig. 232. 



CaHor!jHitef!Jew''tti Hall. Silurian (Niagara Group); Lockport, New- 

 York. A, Caly-^c from cme .side (natural .size). B, Ambulacral grooves 

 and three pectinated rhombs (/■/(), enlarged ; o, Mouth ; (t?i, Anus ; g, 

 Genital pore (after Hall). 



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 suggestion 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 pro- 

 trusion of tube-feet, since 

 they are frequently covered 

 over by an outer calcareous 

 membrane, which effectu- 

 ally shuts off communica- 

 tion with the exterior. 



The arms or brachioles 

 in the Cystideans are as 

 a I'ule feebly developed, 

 and are sometimes either 

 entirely wanting, or re- 

 duced in number (2, 3, 6, 

 9-13). The pentamerous 

 symmetry, so generally 

 characteristic of Echino- 

 derms, 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 pinnules than 

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

 the Canadian genera, Amygdalocijstites and Malocystites, the arms are either 

 recumbent with their dorsal side facing the calyx, or they are prostrate and 

 incorporated into the calyx. The ventral side, in these cases, is directed out- 

 wardly, 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. 



The stem, as a rule, is greatly abbrevi- 

 ated, and is frequently obsolete. Some- 

 times the calyx is attached by the entire 

 lower surface (Agelacrimis) ; or in other 

 cases by means of a tubular 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 overstride one another like the draw-tubes of a spy-glass. In other cases 

 the up})er part, and occasionally, indeed, the entire stem, is composed of vertical 

 rows of alternating plates. These plates, as a rule (Dendroq/stites), enclose 

 a large central s})ace, which may be regarded as a prolongation of the Itody- 

 cavity. 



a, Aristorysfites. Suhtegminal ambulacral 

 grooves ; b, Same of Pyrovi/stitet:. Enlarged (after 



Barrande). 



