ITS STRUCTURE. 115 



A. hellis, occasionally occupy a cliamber, and divers 

 kinds of Nereidous worms crawl freely tlirongli it. 

 A beautiful specimen is now in my Tank, wliick lias 

 OTOwn like a noble crown around tlie summit of a 

 conical stone, tlie whole being nine or ten inches in 

 height. The basal stone is densely covered with 

 parasitic Zoophytes, and tubicolous Annelides of 

 many species. 



But our admiration of this handsome Coral is much 

 heightened when we know something of its nature. 

 We see that its walls, which are not more than one 

 thirtieth of an inch in thickness, are composed of 

 stony substance, yet very brittle. Closer examina- 

 tion shows that this thickness, small as it is, in- 

 cludes two ranges of cells, which are placed back 

 to back, opening by oval orifices on both sides of 

 the walls. 



Every cell is inhabited (or rather lias heen, for the 

 older ones are dead and vacant before the younger 

 are formed) by an active Polype of the Polyzoan 

 Class, whose head, crowned with a funnel of radiating 

 ciliated tentacles, protrudes from the orifice or is with- 

 drawn into it at pleasm'C. These all are united by a 

 common life ; a common bond of sensation and of 

 nuti'ition connects the whole of the individuals into 

 one compound being. A single Polype, inhabiting a 

 solitary cell, began the colony, which has gi'own by 

 the continual formation of new individuals on every 

 side, as buds grow into branches, which bud again 

 and form a tree. 



Some idea of the populousness of such a commu- 

 nity may be gathered from the following calculations. 



i2 



