352 NATURAL HISTOEY. 



the stone uncovered by the 

 flesh. Among the expand- 

 ed Polypes are seen two 

 which are out of their cells, 

 but their tentacles are not 

 expanded. 



614. It is chiefly by the 

 coral-forming animals of this 

 order that the coral reefs 

 and islands have been built. 

 So immense are the works 



Fig. 273. Mass of Astrea viridis. which large companies of 

 these animals perform here and there, that we may re- 

 gard the changes which they produce as among the most 

 important to which the earth has been subjected, at least 

 since it has been inhabited by man. A large number of 

 the Polynesian Islands, and many of those in the Indian 

 Ocean, have been constructed by these little animals. 

 They are continually building extensive reefs, also, in va- 

 rious forms and in different positions. Off the coast of 

 New Holland there is a coral reef over one thousand 

 miles in length. Great as are the changes now going on 

 from the agency of these little architects, it is supposed 

 that in what may be called the forming ages of our earth 

 they had a still greater agency, in the formation of the 

 limestone rocks which constitute so large a part of the 

 crust of the globe. 



615. There is another order of Polypes called the As- 

 teroida, from the star-like appearance of the tentacles. 

 The Red Corals, the " Organ-pipe Corals," the Sea Fans, 

 etc., belong to this order. Some in this order verge to- 

 ward the sponges. Their habits are, for the most part, 

 so much like those of the other Polypes that I will not 

 dwell on them. 



616. The proper place of the Sponges it is difficult to 

 determine. If they are really animals, they are of the 

 lowest grade, exhibiting not the least signs of sensation. 



