7° ZOOLOGY 
and outside the base of the animal. Some of the septa may 
coalesce in the centre of the body, and thus the columella is 
Fic. 48.—Caryophyllia borealis, Fleming, a simple coral, twice the natural size. 
After Sir Wyville Thomson. 
produced ; this is usually continued as a little pillar pointing 
towards the mouth. The outer wall of the theca frequently 
Fia. 49.—Astraea pallida, Dana, a compound coral in its living condition. 
After Dana, 
bears ridges, the costae, which do not necessarily correspond 
with the septa; the latter may be connected with their neigh- 
bours by horizontal shelves, termed dissepiments. 
