ECHINODERMATA. CYSTIDEA. 



CLASS. CYSTIDEA. 



The Cystideans in general appearance resemble the 

 crinoids. They consist of a calyx and generally a stem, 

 the latter as a rule being very sho i ' The calyx is in 

 most cases more or less spherical or ovate, with a depres- 

 sion at its base for the articulation of the stem. It is 

 formed of plates which vary in number from thirteen to a 

 hundred or more, these are not arranged in a regular 

 manner as they are in crinoids, and at best the calyx 

 shows but little radial symmetry. Arms are present, and 

 vary from three to five in number, but they are very 

 feebly developed or sometimes quite rudimentary; in 

 some forms they are free, but in other cases fused to the 

 calyx. Or they may be represented merely by scattered 

 filaments, articulated to the calyx by sockets on some of 

 the plates. At the summit of the calyx (fig. 23 A) either 

 at or near the centre, there is an aperture (a) from which 



B 



FIG. 23. A, Glyptosphcerites letichtenbergi, from the Ordovician of Eussia. 

 a, mouth covered by oral plates; b, ambulacral grooves; c, socket for 

 the armlet; d, anus; e, ovarian aperture, (after Volborth). B, a 

 few plates of the same enlarged, showing double pores. C, Plates of 

 Echinosphcerites, with pore-rhombs. 



