222 



ZOOLOGY 



ered by a shallow sea, an American Mediterranean. In 

 these waters crinoids existed in vast numbers, and their 

 remains may be found in the rocks over a large part of 

 the country. When this sea was drained, during the 

 Mesozoic, the crinoids mostly died out, leaving com- 

 paratively few representatives. In more modern times 

 many genera and species of crinoids have come into ex- 

 istence, but they mostly show little resemblance to those 

 of remote antiquity, and there is no reason to suppose 

 that the group will ever again recover its ancient glory. 



starfish 4. The Asterozoa, or star animals (Greek, aster, a 



star, from which our English word is little modified), in- 

 clude the starfishes and brittle stars. There are very 

 important differences between these groups, although 

 both have the starlike form, with arms extending from 

 a central disk. In the Asteroidea or true starfishes, the 

 arms are usually five, but may be much more numerous ; 

 they are not sharply marked off from the central disk. 

 The arms present on the under surface ambulacral 

 grooves, with podia (singular, podium) or tube feet. In 



Brittle stars the Ophiuroidea (snakelike animals) or brittle stars 

 there is a round central disk, with long, wormlike arms 

 which curl around objects presented to them. The 

 ambulacral grooves are closed, and the podia have only 

 sensory and respiratory functions. The arms readily 

 snap off, whence the name brittle star. In the ophiu- 

 roids the madreporite is on the oral side of the disk. 

 Although the asteroids and ophiuroids are so easily dis- 

 tinguished, there is a group called Lysophiuroida, found 

 in the Palaeozoic rocks of Europe, which is more or less 

 intermediate between the two, indicating that they had 

 a common ancestor. 



5. The Echinozoa include also two extremely distinct 

 groups, the Echinoidea or sea urchins, and the Holothu- 



