THE BENTHAL 207 



the rock; where they occur on bosses of calcareous algae or fragments 

 of coral, etc., on the neighboring sand flats, they are assembled in 

 clusters and modify each other's growth (Fig. 38). A rich fauna lives 

 among the pearl oysters; coral may compete with them for room; 

 sponges and Alcyonaria are abundant; mollusk-eating starfishes such 

 as Astropecten hemprichii and Pentaceros lincki feed on them; and 

 annelids, bryozoans, and tunicates cover their shells. The nearness of 

 the sandy areas may be fatal to the pearl banks if a storm stirs up the 

 sand and redeposits it on them. 



Coral reefs. — Coral reefs form a special biotope in the rocky- 

 coast formation. These reefs are raised structures, extending from sea 

 bottom to low tide level, formed by living animals and consisting of 

 their calcareous skeletons. Great masses of carbonate of lime are 

 built up here under the favorable temperature conditions of the tropi- 

 cal seas, to which the reef-building corals are confined (cf. p. 271). 



These reefs are formed by animals belonging to a varied group. 

 The Madreporaria (reef corals proper) are the main component; the 

 lime skeletons deposited by their ectoderm form the foundation. Three 

 families of reef corals are particularly important, the Poritidae 

 (Pontes, Fig. 39a) , the Acroporidae (Madrepora, Fig. 39, above) , and 

 the Astraeidae (Gonastraea, Coeloria, Fig. 396). The organ corals 

 among Hexacorallae (Tubiporidae, Fig. 39c) must be added to these. 

 So must the Milleporidae, belonging to another class of the coelen- 

 terates, known to the inhabitants of the Red Sea area as "fire corals" 

 on account of their stinging powers. These form massive branches by 

 calcification of their ectodermal skeleton, an evolution convergent with 

 that of the reef corals. 



The calcareous algae play an important part in reef building every- 

 where; their bulk is composed of carbonate of lime up to 90%. Calcare- 

 ous Bryozoa compete with them in places in growing over and cement- 

 ing fragile or broken coral branches. The calcareous tubes of the 

 serpulid worms play a similar part, sometimes an important one, as in 

 Bermuda. 47 Some sessile mollusks, both lamellibranchs and gasteropods, 

 contribute to the accumulation of lime. Foraminifera, in spite of their 

 minute size, play a not unimportant role in supplying binding material 

 to cement the fragments and fill the holes. 



The reef corals are stenothermal, warmth-limited animals, and 

 coral reefs are confined to warm seas. They flourish only in water 

 above 20.5°, which is almost confined to the tropics, approximately 

 between 30°N. latitude and 30°S. (cf. map, Fig. 75). The Gulf Stream 

 enables them to reach and form reefs at Bermuda in latitude 32°N. 

 Even within the tropics, areas of cold up-currents as on West Africa 



