THE ECHINODERMATA 243 



PELMATOZOA. 



Cystidea. Extinct, Cambrian to Permian, but mainly 

 Lower Palaeozoic. 



Blastoidea. Extinct, Silurian to Permian, but 

 mainly Upper Palaeozoic. 



Edrioasteroidea. Extinct, Ordovician to Carbon- 

 iferous. 



Crinoidea. " Sea-lilies," Ordovician to Recent. 



ELEUTHEROZOA. 



Echinoidea. " Sea-urchins," Ordovician to Recent. 

 Stelleroidea. " Star-fish," Cambrian to Recent. 

 Holothuroidea. " Sea-cucumbers," Cambrian to 

 Recent. 



All echinoderms are marine. 



It will be convenient to describe first some examples 

 of crinoids. 



i. Cupressocrinus gracilis (Fig. 67) is one of many 

 crinoids found in the Middle Devonian " crinoid-bed " of 

 Gerolstein in the Eifel (Rhenish Prussia). It consists of 

 a root, stem, and* crown. By the first (not often preserved) 

 it was fixed to the sea-bottom, but this "root" had no 

 absorbent function like the root of a plant. The stem 

 lifted it up high above the mud : as preserved fossil 

 it consists of a large number of squarish discs piled up 

 into a column : these are the stem-ossicles or columnals. 

 Through each of them runs a central vertical tube (axial 

 canal) and four smaller peripheral canals. Adjaceyt 

 columnals were united, in life, by organic tissues, and the 

 surfaces of contact were roughened to give greater grip 

 or " key " : on the articular surface of an isolated ossicle 



