120 THE WEALTH OF THE SEA 



ocean by these minute polyps which can scarcely 

 be seen by the naked eye, it is extremely difficult to 

 conceive of the tremendous numbers of the little 

 organisms that lived and died in order to rear these 

 beautiful monuments. The huge coral masses com- 

 monly known as madrepores, out of which coral 

 islands and reefs are constructed, all commence as 

 a single coral animal which grows by budding; the 

 buds remain attached to the parent, which soon dies, 

 leaving its limestone skeleton. 



Biologically, an ordinary coral animal, or polyp, 

 does not differ from a sea-anemone except for the 

 possession of a calcareous skeleton secreted within 

 its basal tissues. The sea-anemones and corals are 

 almost the lowest organized group of living animals. 

 The body is really a simple sac, the orifice of which 

 is inverted and held in position with relation to the 

 outer wall by a series of radiating partitions. One 

 or more rows of tentacles, varying in number and 

 character according to the species, surround the 

 mouth of the sac. It does not possess a distinct intes- 

 tinal tract ; the whole space inclosed within the outer 

 wall contains the digestive juices. The reproducing 

 elements are located on the surfaces of the radiating 

 membranous partitions, which also, in the case of the 

 corals, partly secrete the symmetrical radiating cal- 

 careous plates. 



The form and color of colonies of coral animals 

 differ widely, depending on the species. Most of the 

 common names of the various species are derived 

 from either the form or color of the colonies, some 



