SILURIAN AXIMALS. 



311 



If, now, we draw the diagonal, C B, then the shaded triangle would 

 represent the stemmed, and the unshaded the free, and the diagonal the 



Fig. 325.— A Living Free Crinoid— Comatula rosacea, the Feather-Star: a, free adult; b, fixed young 



(after Forbes). 



line of decrease of the one and increase of the other ; and the whole 

 figure the general relations of the two sub-classes throughout time. In 



PALAEOZ O IC -— 



ilurictn Devon? Caroonif 1 



NEOZOIC 



STEM M ED 



Fig. 326.— Diagram showing the Distribution in Time of the Class of Echinoderms. 



the Palaeozoic the stemmed predominate ; in the Mesozoic the two are 

 equally represented ; in modern times the free predominate. 



Stemmed Echinoderms, or Crinoids, may be divided into three fami- 

 lies, viz. : 1. Crinids ; 2. Cystids ; 3. Blastoids. Crinids are the typical 

 Crinoids, with branching arms, already illustrated from living examples 

 (Figs. 323-325). Cystids (Figs. 327-330) are of a Madder-like form 

 (hence the name), and are either without arms, or else have few, short, 

 simple arms springing from near the center of the upper part of the 

 body, the mouth being probably on one side. The radiated structure 

 in these is imperfect. Blastoids (Gr. /3Aa<rro?, a bud) had a bud-shaped 

 body, with five petalloid spaces (ambulacra) radiating from the top and 



