The coelenterata or Cnidaria have taken a step beyond the simple sponge, developing bodily tissues for different functions. The living coral, a tentacle-ringed polyp, has built this cone-shaped skeleton for its support.
Most corals are symbiotic with green algae that live inside (living together to the advantage of each). Without the algae, the coral can’t grow. Without sunlight, the algae can’t grow. So most corals live only in relatively shallow, clear water where sunlight penetrates. Corals also require relatively warm water of normal salt content. Thus, the presence of corals has told us something about this ancient Kaibab sea!
Many corals are colonial, forming large masses and reefs. But the horn coral is solitary, a single polyp and its skeleton.