342 ECIHNODERMA. 



Sub Class I. Eucrinoidea. 



The foregoing account applies entirely to the Eucrinoidea, which may 

 be divided into two groups : 



Order I. TESSELLATA (Palseocrinoidea). Theca 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, 

 tneca with compact, in part movably articulated, plates. This order left 

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

 Rhizocrinus* (fig. 323) and Pentacrinus (fig. 320), deep seas ; the COMA- 

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



Sub Class II. Edrioasteroidea (Agelacrinoidea). 



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

 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, 

 Echinosphcerites (fig. 326). 



FlO. 326. Fio. 327. 



FlO. 328.Echinosphceritesaurantium. (From Zittel ) 



FIG. 3SR.Pentremitesflorealis. (From Zittel). Lateral, oral, and aboral views. 



Sub Class IV. Blastoidea. 



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

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

 iferous. Pentremites (fig. 327). 



