THE REEF-BUILDING CORALS. 149 



members must be grouped either with the Hexacoralla, the 

 Octocoralla, or the Hydrozoa. 



The Rugo&a constitute a group of extinct and mainly 

 Palaeozoic stone-corals, the thecae of which are provided with 

 tabular dissepiments, and generally have the septa less de- 

 veloped than those of the ordinary stone-corals. The arrange- 

 ment of the parts of the adult Mugosa in fours, and the 

 bilateral symmetry which they sometimes exhibit, are inter- 

 esting peculiarities when taken in connection with the te- 

 tramerous and asymmetrical states of the embryonic Hexaco- 

 ralla. On the other hand, some of the Mugosa possess oper- 

 cula, which are comparable to the skeletal appendages of the 

 Alcyonarian Primnoa observed by Lindstrftm, and the te- 

 tramerous arrangement of their parts suggests affinity with 

 the Octocoralla. It seems not improbable that these ancient 

 corals represent an intercalary type between the Hexacoralla 

 and the Octocoralla. 



All the Actinozoa are marine animals. The Actinia?, 

 among the Hexacoralla, and various forms of Octocoralla, 

 have an exceedingly wide distribution, while the latter are 

 found at very great depths. 



The stone-corals, again, have a wide range, both as respects 

 depth and temperature, but they are most abundant in hot 

 seas, and many are confined to such regions. Some of these 

 stone-corals are solitary in habit, while others are social, grow- 

 ing together in great fields, and forming wdiat are called 

 " coral reefs." The latter are restricted within that compara- 

 tively narow zone of the earth's surface which lies between 

 the isotherms of 60°, or, in other words, they do not extend 

 for more than about 30° on either side of the equator. It is 

 not conditions of temperature alone, however, which limit 

 their distribution ; for, within this zone, the reef-builders are 

 not found alive at a greater depth than from fifteen to twenty 

 fathoms, while at the equator, an average temperature of 68° 

 is not reached within a depth of 100 fathoms. 



Not only heat, then, but light, and probably rapid and 

 effectual aeration, are essential conditions for the activity of 

 the reef-building Actinozoa. But, even within the coral zone, 

 the distribution of the reef-builders appears to be singularly 

 capricious. None are found on the west coast of Africa, very 

 few on the east coast of South America, none on the west 

 coast of North America ; while in the Indian Ocean, the Pa- 

 cific, and the Caribbean Sea, they cover thousands of square 



