343 



ECniNODERMA. 



Sub Class I. Eucrinoidea. 



The foregoing account applies entirely to the Eucrinoidea, which may 

 be divided into two groups : 



Order I. TESSELLATA (Palfeocrinoidea). Theea with its side walls 

 composed of immovably united thin plates ; the ambulacral grooves usu- 

 ally completely covered by calcareous plates. Exclusively paleozoic. 



Order II. ARTICULATA (Neocrinoidea). Ambulacral grooves open, 

 theca witli compact, in part movably articulated, ]jlates. This order left 

 the other in the mesozoic age, and some families have persisted until now. 

 Rhizoarinus* (fig. 323) and Pejitacrinus (fig. 320j, deep seas ; the Coma- 

 TULID.E are fixed in the young, free in the adult. Antedon* (fig. 321). 



Sub Glass II. Edrioasteroidea {Agelacrinoidea). 



Theca of irregular plates ; arms unbranched and lying on the theea. 

 Possibly the ancestors of the noncrinoid echinoderms. Paleozoic. Agela- 

 crinus. 



Sub Class III. Cystidea. 



Exclusively paleozoic ; body spherical, composed of polygonal plates. 

 Stalk and arm structures rudimentary, sometimes lacking. The Ampho- 

 RiDA are by some regarded as ancestral of all echinoderms. Holocystites, 

 Echinospliarites (fig. 336). 



Fig. 336. Fig. 327. 



Fig. 320.— -Ec/uno.'.p/ja'r/Yc.'; aura-ntiuni. (From Zitti'l ) 



Fig. 327.— P'^virefnUesJhiiPfili.'i. (From Zittcl). Lateral, oral, and alioral views. 



Sub Class 71'. /t'lastnidca. 



Arms lacking ; the mouth surrounded by five petal-like ambulacral 

 areas. The group appears at end of Silurian and dies out with the earbon- 

 iferous. I'cittreniites (fig. 327). 



