GENERAL PRINCIPLES 



17 



wholly absent or only slightly developed; the Blastoids are bud- 

 like forms with plates of the shell in very regular radial order, and 

 without arms; and the Crinoids are lily-like forms with regular 

 radial arrangement of plates of the shell, and with long, feathery 

 arms surrounding the mouth. 2. Aster moans are the free-moving, 

 star-shaped Echinoderms usually with five arms or rays radiating 



Fig. 7 

 A modern Asterozoan ("Starfish"). 

 duction to the Study of Fossils,' 

 millan Company.) 



(From Shimer's "Intro- 

 permission of The Mac- 



from a central disk. Of these the Ophiuroids (Brittle-stars) have 

 slender, flexible rays very distinct from the central disk, while the 

 Asteroids have thicker rays not so sharply separated from the 

 central disk, and the alimentary canal extends into the rays. 

 3. Echinozoans are not free-moving, are without free arms, and 

 are stemless. Of these the Echi?ioids (Sea-urchins) have hard 

 shells made up of calcareous plates usually immovably joined and 

 covered with numerous movable spines; and the Holothuroids 



