HISTORY OF THE EARTH 



297 



(4) ECHI'NODERMS. 



Under this name are included such animals of the sea 

 as starfish, sea urchins, and crinoids. They possess 

 distinct nerves, digestive organs, and a circulatory 

 system, in which the fluid is water instead of blood. 

 These features mark a notable advance over the 

 polyps. Most echinoderms are protected by a hard 

 shell composed of a mosaic of little plates. 



(a) Crinoids. 



Often called sea lilies, because they are attached 



to the sea floor by stalks, while the upturned 



mouth is surrounded 



by long, feathery arms 



(Fig. 364). Cystids and 



blastoids (Fig. 390) are 



related forms. 



FIG. 307. A modern sea 

 urchin with the spines at- 

 tached. 



(6) Starfish (including the 

 brittle stars). 

 On account of their star- 

 like shape, these animals 

 will not be confused with 

 any others. 



(c) Sea urchins (Echinoids'}. 



Round or egg-shaped forms which in life are 

 covered with spines (Figs. 307 and 444). They 

 are very abundant to-day along the seashores, 

 but are less important among fossils. 



(5) WORMS. 



A large group of animals which are still more complex 

 in structure than the echinoderms. As they are soft- 

 bodied they are not commonly preserved in the rocks, 

 but their burrows in the sand are often found as tubes 

 in sandstone. 



