98 



CORAL. 



of some extinct volcano, for they are almost always of a circular 

 form, with a lake in the centre communicating with the ocean 

 by a single channel : some are more than ten leagues in dia- 

 meter. 



19. Almost all polyps inhabit the sea: some, however, are 



found in fresh water. 

 Most polyps secrete this 

 stony matter, above 

 mentioned, in the cells 

 of which they are lodg- 

 ed, or around which 

 they are grouped. The 

 stony matter, of a beau- 

 tiful red colour, employ- 

 ed as an ornament, call- 

 ed coral, is formed in 

 this way ; it is the stem 

 found in the midst of 

 an aggregation of cer- 

 tain polyps, that serves 

 to sustain and attach 

 them to the earth (jig. 

 $9). These tittle ani- 

 mals, onFy two or three 

 lines in length, have at 

 their free extremity 

 eight tentacles, in the 

 middle of which is the 

 mouth ; by their oppo- 

 site extremity they are 

 fixed in little cavities hollowed out in a kind of membrane or 

 living bark, which is common to all, and into which they can 

 entirely withdraw themselves ; this common part is more or less 

 branched, and in its centre are found successive layers of very 

 hard, stony matter, ^vhich is the coral. This coral rs found 

 plentifully in the Mediterranean, principally on the African coast, 

 where it forms the object of an active fishery. 



20. Fresh-water polyps (fg. 90) or Hydrce (from the Greek, 

 'udar, water) may be considered as the most simple type of this 

 group. The body is a gelatinous tube, in which no particular organ 

 is perceived ; nevertheless they crawl and swim actively, by agi- 

 tating their long tentacles, to seize small animals that come within 

 their reach, which they devour with great avidity ; they seem to 

 be sensible to the influence of light. Some of these polyps have 



19. What is red coral ? Where is it found? 



20. What are hydras ? Where are they found ? 



Fig* 89, CORAL. 



