CALCAREOUS POLYPIERS. 195 



12. The more active submarine constructors - Astroides — Caryo- 

 phillia— Madrcpora Plantaginea — Dcntlrophyllia — Occulin:i, or 

 Wliite Coral — Mean Irina— Fungia — Porites— Milleporaj. 



The purely calcareous polypiers have their prin- 

 cipal seat in tropical regions. These are tlie species 

 which exercise the most marked influeiiccj upon the 

 varied surface of the sea-bottom, and which have 

 distinguished themselves as the constructors of reefs 

 and islands. The principal types of this class of 

 polypiers are the caryophillia^ the meandrina, the 

 fungia, and the pentacrini. For a long time tht se 

 species were confounded together under the gent ml 

 name of madrepores, and it is only during late years 

 that their polypi have been recognised. 



These polypi bear a very close resemblance to 

 thosf* of actiniae and corals; but the foldings which 

 we have remarked in the digestive cavity are not 

 connected, as is the case in the latter. They generally 

 occur at the base, so as to constitute a central axis 

 surrounded with ra liating cells, which are only quite 

 separated from each other at the lower part of the 

 animal. Their similarity to the actiniae, already 

 described, is apparent at first sight. In proportion 

 as the polypier grows, its lower part becomes cal- 

 careous, and reproduces in stone the soft structure 

 that had bei n formed by the living animal. The 



