CORALS. 125 



CORAL ISLANDS. 



The study of the formation of coral islands is one of the most fascinating connected 

 witli the Actinozoa. So long as naturalists limited their attention to the coral life of 

 the Mediterranean and other European seas, where no great coral reefs exist, this part 

 of the subject attracted but little attention, but with the awakening of interest in all 

 branches of natural science at the beginning of the present century, more especially 

 at tlie time of the great exjsloring expeditions sent out to the tropics by the different 

 go^•ernments, the Ijuman mind naturally turned to these islands, and the mode of their 

 formation became a subject of scientific interest. Tlie subject has a geological as well 

 as a zoological side, and touches on several questions of physical geography. It is, 

 moreover, almost incomprehensible without a clear idea of the chemical nature of the 

 solid lime secretions of the coral body, which plays the most important part in the 

 origin and growth of these characteristic islands. 



While the present subject is considered in relation to the coral animals, it must 

 not be su])posed that they alone make the coral reefs. Although most conspicuous in 

 this woik, there are others of very great importance in their formation from the time 

 the foundations are laid on the sea floor up to that when the reef emerges from the 

 waves in the form of an island. Of these might be mentioned a few genera of stony 

 hydroids as 3Iillepora, molluscan shells of all kinds, Bryozoa, Radiolaria, pelagic 

 Protozoa, and genera of marine Algse. Many instances of coral islands foi-med in part 

 liy any of the last mentioned, will no doubt suggest themselves. A few examjiles will 

 suffice for illustration. Cooper's Island, one of the large coral islands of the Bermuda 

 group, has a sandy beach which is almost wholly made np of oceanic Foraminifera. 

 The beach adjacent to the landing at Fort Jeffei-son, on the Tortugas, Florida, is w^holly 

 formed of fragments of a large Nullijiore, an Alga which secretes lime in its tissues, 

 and is found alive in large clumps in the moat about the fort. Shelly Bay at the 

 Bermudas is composed almost entirely of fragments of lamollibranchiate shells ground 

 into a fine sand. Strata of rock formation in the Bermuda of four or five inches in 

 thickness are made up entirely of the shells of land Helices, some of great size, firmly 

 cemented together. While these and many other animals contribute to the formation 

 of the coral islands, corals seem essential to their growth, for we find characteristic coral 

 reefs confined to the zone which by the thermal conditions of the water limits the 

 home of these animals. 



The distribution of reef-building corals follows a number of most interesting laws. 

 In latitude their home is limited north and south of the equator by the water isotherm 

 of 68° Fahr. These lines projected on our globes follow no parallels of latitude, often 

 being widely sejiarated from the equator and then ajjjiroaching to its immediate 

 vicinity. While the reef-building corals are limited to this zone it must not be 

 su])])osed that all genera of Actinozoa are hemmed into these narrow limits. Many 

 corals, some of which secrete a calcareous skeleton, are found in all latitudes as fai- as 

 naturalists have explored the marine life. 



The distribution at present of coralline life on the globe is very different from what 

 it has been in the past. While reef-building corals now never ventm-e into latitudes 

 higher than 35°, the evidence drawn from fossil coral banks shows that in older times 

 they were found in very high latitudes. In the North Atlantic Ocean at the present 

 time the northern limit of extensive banks of coral is the lonely Bermuda grouji in the 

 latitude of 32° N. 



