Phylum VIII. ECHINODERMATA. 



Branch Pelmatozoa. 



Class Cystoidea von Buch. 



The cystoids are entirely extinct marine invertebrates which 

 flourished only during Palaeozoic time. Most of them lived during 

 the Ordovicic or Siluric eras, but Cambric and Carbonic forms are 

 also known. They were mostly stemmed organisms with a calyx 

 like the crinoids, but the arms were imperfect and a few of them 

 were stemless. The calyx, which varies in form, is composed of 

 polygonal plates which are united by close sutures. The plates 

 vary in number in different species, from thirteen to several hun- 

 dred, and only exceptionally exhibit a regular arrangement. A 

 radial arrangement of plates, like that of the crinoids, occurs 

 rarely, and the side plates pass insensibly into the plates of the 

 ventral (upper) side. In the center of the dorsal (under) side, 

 however, a regular series of basal plates exists, which rest on the 

 stem or column. 



The mouth is indicated by a central or nearly central aperture 

 on the upper {ventral) surface and is sometimes covered by small 

 plates. From it radiate two or more simple or branching amhu- 

 lacral grooves or ambulacra, which are also frequently roofed over 

 by plates (Fig. 1776). From the distal end of the ambulacra arise 

 the arms. These are feebly developed in the cystoids and are often 

 entirely absent. When present, they are unbranched, consisting of 

 a single (uniserial) or a double (biserial) row of plates, and pos- 

 sess a ventral groove, protected by covering plates. Just beneath 

 the mouth is often a small porous plate, the madreporite. 



More excentrically situated than the mouth is the anal opening; 

 this is frequently closed by a valvular pyramid. 



The calyx plates in most cystoids are perforated by pores or 

 fissures. These are often arranged to form lozenge-shaped or 

 rhombic figures, the pore-rhombs, which are disposed one half on 

 each of two adjoining plates, while the line of suture between the 

 plates forms either the longer or shorter diagonal of the rhomb 



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