478 RADIATA: POLYPI. 



or lobes. They are free, and effect locomotion by sliding 

 along by means of their fleshy " foot." Most kinds are 

 simple. And nearly all are soft throughout; a few secrete 

 from the base a horn-like substance. 



Other actinarians, as the Antipathus group, are com- 

 pound communities, and appear in the form of delicate 

 shrubs and twigs, attaining a height of three feet in 

 some cases. They secrete a horny axis, which is covered 

 by the united polyps, which have tentacles similar to a 

 Sea-anemone. 



Still other actinarians, as the Zoanthids, live in com- 

 pound communities in most cases, but, like the Sea-ane- 

 mones, secrete no coral. They are unlike the latter in 

 being incapable of locomotion. The polyps have simple 

 short tentacles on the margin of the disk. 



SUH-SECTIOJST IV. 



THE ORDER OF MADREPORARIA OR MADREPORES, 



ANS, ETC. 



THE polyps of this group are simple or compound, 

 often excessively branching, and they form coral in their 

 walls or outer parts, in their radiating partitions, and 

 often at their base. The forms which the communities 

 assume are very beautiful and exceedingly various, and 

 are among the most beautiful objects in zoological cabinets. 



The great group of Madreporacea contains polyps which 

 have a definite number of tentacles, twelve or more. 

 Those called Porites (Fig. 723), have the cells shallow, 

 and not more than one-twelfth of an inch in diameter, 

 and the coral in some cases branching, in others massive. 



