CYSTIDEA. 



305 



BT-. 



The body consists of a calyx which is either prolonged aborally into a 

 stalk (Fig. 215, etc.) or is without a stalk (Fig. 211, etc.) In the latter case 

 the body was probably attached by its aboral surface to the substratum. 

 When a stalk is present it frequently has rather the appearance of a taper- 

 ing aboral continuation of the body (Fig. 212) than of a sharply differen- 

 tiated stem like that of the stalked Crinoids. 

 The stem, which is often very short and with- 

 out cirri or roots, does not, as a rule, appear to 

 have served for attachment. It is frequently 

 coiled. The calcareous plates of the body wall 

 are usually numerous and irregularly arranged ; 

 but sometimes they are larger, few in number 

 and arranged in definite cycles (e.g. Cystoblastus 

 Fig. 213). They are united by sutures. There 

 is as a rule no sharp line of demarcation between 

 the oral and aboral surfaces, or between the 

 plates of the radial and interradial areas. 



The mouth is at or near the centre of the oral 

 surface and is sometimes covered by oral plates. 

 In the simplest forms ambulacral grooves are not 

 visible (Figs. 212, 214) and no radial structure 

 can be made out, but it is asserted by Barrande 

 that such forms probably have subtegminal 

 grooves. Usually ambulacral grooves are pre- 

 sent, and they are placed either on the surface 

 of the calyx (Fig. 217) or on processes of the 

 edge of the mouth (Fig. 216). They vary in 

 number from two to five and may branch. They 

 frequently possess covering plates (Fig. 219) 

 which in life must have been capable of being 

 folded back so as to expose the groove. Arms 

 are often quite absent, and when present are 

 usually small. They either project from the 

 edge of the mouth (Fig. 216) or further out from 

 the calyx (Fig. 220). The arms vary in number 

 from two to thirteen. In some cases the so- 

 called arms resemble pinnules, as in Glypto- 

 sphaera, Protocrinus (Fig. 217) in which the 

 ambulacral grooves branch and end in small 

 arms : these may perhaps be called brachioles, 

 though it is often impossible to settle whether to 

 apply the term brachiole or pinnule to an arm- 

 like process. Undoubted pinnules arise from the 

 edges of the ambulacral grooves in some cases 

 and the ambulacral grooves are continued on to 

 them (Fig. 215). The anus is on the same sur- 

 face as the mouth but excentrically placed in 

 an interradius (Figs. 211, 217, etc.) Between it 

 and the mouth two openings can in some cases 

 be made out : one of these, that nearest the anus, is interpreted as the 

 genital opening ; the other is supposed to be the water-pore (Fig. 214). 

 If the interpretation of the first of these openings is correct, it would 

 appear that the genital organ of Cystids is in the calyx and is not radially 



z in. x 



-Bi 



/--St 



FIG. 212. Dendrocystis 

 Sedgwifki (after Barrande 

 from Bather). As anus 

 Br the arm-like appen- 

 dage ; st stem. 



