Final answer:
The radial ridges within coral depressions are called septa, which supported the soft tissues of the coral polyp. These septa extend from the columella and are key structures within the corallite, a chamber that houses individual polyps.
Step-by-step explanation:
The radial ridges within each depression on a piece of coral are the physical remnants of structures called septa, which supported the soft tissues of the coral polyp.
These septa resemble the spokes of a bicycle wheel, radiating outwards from a central pillar-like structure called the columella.
During the coral polyp’s growth, septa are added in a pattern that ultimately divides them into four quadrants, each representing a space between septal ridges.
In certain varieties of corals, such as rugose corals, these spaces are referred to as fossula.
The corallite is the compartment within the coral colony that houses individual coral polyps.
The septa provided necessary structural support for the coral's soft tissues within these chambers.
Over time, as polyps grew upwards, they also developed horizontal partitions called tabulae, further contributing to the structural integrity and compartmentalization of the living space within the coral.