ACTINOZOA. 171 



principal share in the formation of coral reefs live near the surface, 

 being rarely found alive at a greater depth than 40 fathoms. They 

 are confined to a zone extending about 28 degrees on either' side 

 of the equator, and only here and there extend beyond these 

 bounds. Their calcareous skeletons, together with those of mille- 



Fio. 143. Astraea (Goniastraea) pectinata FIG. 144. Maeandrina (Coeloria) arabica Klz. 

 Ehrbg. (after Klunzinger). (after Klunzinger). 



pores, the shells of molluscs, echinoderms, annelids, foraminifera, 

 cemented together into a compact rock by encrusting organisms and 

 by deposited lime, build up in the course of time masses of colossal 

 extent. 



Coral reefs are generally met with in one of three forms, fringing reefs, barrier 

 reefs, and atolls. 



A fringing or shore reef is a platform of rock skirting the shore and ending, 

 seawards, in an abrupt edge from which there is a steep slope down to the sea 

 bottom. To a depth of 20 or 30 fathoms the slope is covered with growing 

 coral, the upper part being bathed in the surf of the breaking rollers. The 

 upper surface of the outer edge of the reef, which is generally uncovered at low 

 water, is higher than the part nearer the shore, and, in exposed reefs, is largely 

 formed by encrusting calcareous algae (Nullipores), which thrive in the freshly 

 aerated water. 



In reefs in protected situations some corals, especially the Madrepores, grow 

 on the edge of the reef wh ere they are laid bare at low tide. The reef is frequently 

 broken here and there by channels which have a depth of a few fathoms and 

 run in towards the land in branching and tortuous courses, often expanding 

 into irregular pools floored by coral sand. These protected pools and channels, 

 whose vertical sides are formed by a luxuriant growth of hard corals, millepores, 

 and the leathery colonies of Alcyonaria, are tenanted by an abundant reef fauna 

 often displaying the most brilliant and varied colours. Between the channels 

 the surface of the reef is formed of beds of corals which have nearly reached 

 the surface, or of tracts of dead coral-rock, or sand often consisting largely of 

 Foraminifera. 



Nearer the land the growth of coral becomes less abundant and the water- 

 deepens, forming a shallow channel which at high water may be practicable for 



