ACTINOZOA. 



89 



meet the columella (when this is present). These are called the 

 "primary septa." Others, however, fall short of the centre by a 

 greater or less distance ; and these are called the " secondary " and 

 "tertiary" septa, according to their breadth (flg. 51). 



In accordance with their mode of formation, an ordinary com- 

 pound sclerodermic coral may be distinguished from a sclerobasic 

 coral by the fact that it would show a number of little cups or 

 calices, in which the polypes were contained, whereas these cups 

 would be absent in the latter. In accordance, also, with the funda- 

 mental character of the order Zoantharia, the corals of the present 

 group possess septa which are some multiple of dx. 



"When it is "understood that compound corals, such as we have been speaking 

 of, are produced by the combined efforts of a number of polypes, essentially 

 the same in structure as our ordinary sea-anemones, it is readily intelligible 

 that under favourable circumstances large masses of coral may be produced in 

 this way. When these masses attain such a size as to be of geographical im- 



Fig. 52. — Structure of Coi-al-reefs. 1. Fringiug-rcef ; 2. Barrier-reef; 3. Atoll: a Sea- 

 level ; h Coral-reef ; c Primitive land ; d Poi-tiou of sea within the reef, foniiing a 

 channel or lagoon. 



portance, they are spoken of as "coral-reefs," and the phenomena exhibited 

 by these are of such interest as to demand some notice. The coral-producing 

 polypes require for their existence that the average temperature of the sea 

 shall not be less during winter than 66°; and there are therefore no coral-reefs 

 in temperate seas. Keefs, however, abound in all the seas not far removed 

 from the equator, being found chieily on the east coast of Africa and the shores 



