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 ( fig. 

 89). These little ani- 

 mals, only 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, which is the coral. This coral is 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, 

 'udor, 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? 

 JO. What are hydras ? Where are they found ? 



Fig. 89. CORAL. 



