296 HISTORICAL GEOLOGY 



(3) POLYPS (Ccelenterates). 



Animals having a central cavity which serves as a 

 stomach, and a series of radiating arms or tentacles 

 around the mouth. They are all aquatic and most of 

 them live in colonies attached to each other. Many 

 resemble plants in their general appearance and sta- 

 tionary habits of life. The classification of this group 

 is complicated, but the following divisions are of special 

 interest. 



(a) Jellyfish (Medusce). 



Floating jellylike animals (Fig. 306) of great 

 beauty and interest, but de- 

 void of hard parts and there- 

 fore rarely preserved as fossils. 



(6) Hydroids. 



Delicate plantlike animals, 



most of them without hard 



parts. Only the extinct group 



known as the graptolites are 



common in the fossil state. FlG . 306 ;_ A ' modem 



They were minute polyps at- jellyfish or medusa. 



tached in rows to long stems ; 



in appearance the fossils resemble serrate blades of 



grass (Figs. 346 and 347). 



(c) Corals. 



Most of these polyps are provided with a limy 

 shell, exuded by the outer side of the body, and 

 are therefore readily fossilized. In some species 

 each animal lives by itself and leaves a horn-shaped 

 skeleton (Fig. 348), but the majority are attached 

 to each other in the form of compact or branching 

 colonies (Figs. 349 and 372). The young corals 

 swim freely in the water, but become fixed to the 

 sea bottom before they reach maturity. 



