30 ON POLYPS. 



face so as to form innumerable cells or lodges containing the 

 polyps which provide nourishment for the general mass, we should 

 have a good general idea of the structure of the tribe of polyps 

 which now comes beneath our notice. 



The shallower parts of the tropical seas contain countless forms 

 of madrepores, known to us, unfortunately but too often, only by 

 the earthy skeletons which the beauty of their appearance induces 

 the mariner to bring to our shores. These calcareous masses 

 assume more or less an arborescent appearance, spreading to a 

 considerable extent, so as to cover the bottom of large tracts of the 

 ocean, and not unfrequently they play an important part in pro- 

 ducing geological changes which are continually witnessed in the 

 regions where they are abundant. 



(41.) The extent of our knowledge of the animals themselves is, 

 unfortunately, but very limited. That the entire skeleton, whatever 

 its form, is encrusted with living substance ; that the cells contain 

 polyps, resembling more or less those of the aleyonidse, and which 

 provide for the nutrition of the whole, is pretty much the extent 

 of our information concerning them : and should the scientific 

 naturalist ever be placed in circumstances where he can more closely 

 examine them in their living state, there is scarcely a department 

 of science in which his labours could be more beneficially employed 

 than in the investigation of their structure and history. 



(42.) That the madrepores, from the immense masses of chalky 

 material which they accumulate in the regions inhabited by them, not 

 unfrequently become the cause of excessive danger to the mariner, 

 by raising the bottoms of the shallow seas which they frequent, so 

 as to render regions once covered with deep water no longer navi- 

 gable, or filling up by their accumulation the bays and harbours of 

 the South Seas, is undeniable; and a knowledge of this fact justly 

 makes the navigator cautious in passing through the localities where 

 they most abound. Yet the imagination of authors has not seldom 

 far exceeded the truth in detailing the circumstances connected with 

 them. That the harbour of Tinian, so extolled in the Voyages of 

 Lord Anson and others, is now choked up with the skeletons of 

 madreporegynous polyps, is readily credited ; that islands are gra- 

 dually formed, where none existed, by the agency of these creatures, 

 is equally authenticated ; and that madrepores are found in strata 

 much elevated above the level of the seas in the neighbourhood, is 

 a fact attested by many voyagers. Yet when we are told of coral 

 reefs, some hundred miles in length, entirely formed by the agency 



