ACTINOZOA. 95 
spring from the inner surface of the “theca” (fig. 35), and ad- 
vance towards its centre. Often the septa unite centrally with 
a calcareous pillar or rod, the ‘columella ;” and they are of 
different breadths. A certain number of them extend quite to 
the centre of the coral, where they meet the columella (when 
this is present). These are called the ‘“‘ primary septa.” Others, 
however, fall short of the columella by a greater or less dis- 
tance; and these are called the ‘“‘secondary” and “tertiary” 
septa, according to their breadth (fig. 35). 
Fig. 35.—Diagrammatic sections of corals. A, section of sclerodermic coral, 
showing five primary septa and the columella. B, Section of rugose coral, 
showing four primary septa and the columella; ¢ Ridges (‘‘coste”’), formed by 
the septa externally. 
In accordance with their mode of formation, an ordinary com- 
pound sclerodermic coral may be distinguished from a sclero- 
basic 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 fundamental character of the order Zoantharia, the 
corals of the present group always show septa which are some 
multiple of five or szx. 
When it is understood that compound corals, such as we have been 
speaking of, are produced by the combined efforts of a number of pol- _ 
ypes, 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 importance, 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 as our seas are considerably colder than this, we 
have no coral-reefs. Reefs, however, abound in all the seas not far re- 
