ECHINODERMS {SPINY ANIMALS) 



41 



Fig. 67.— 



Crinoid, 



arms closed. 



The crinoids are the most ancient of the echino* 

 derms. (Figs. 67, 68.) Their fossils are very 

 abundant in the rocks. They 

 inhabited the geological seas, 

 and it is believed that some of 

 the other echinoderms de- 

 scended from them. A few 

 now inhabit the deep seas. 

 Some species are fixed by 

 stems when young, and later 

 break away and become free- 

 swimming, others remain 

 fixed throughout life. 



The four classes of the branch echinoderms are 

 Starfish {asteroids), Sea urchins {echincids), Sea 

 cucumbers {holothurians), and Sea lilies {crinoids). 



Comparative Review 



Make a table like this as large as the page of the 

 notebook will allow, and fill in without guessing. 



Fig. 68. — Disk of Cri- 

 noid from above, show- 

 ing mouth in centre 

 and vent near it, at 

 right (arms removed). 



