II.

What Are Corals?

The basic form of a coral animal is called a polyp. A polyp has a tubular body with one end attached to or resting on the sea bottom, or the polyp may be attached to a coral reef. At the other end is a single opening, a mouth, which is surrounded by tentacles. The main structure of the body consists of only two layers of cells, an outer wall or skin called the epidermis and an inner wall known as the endodermis or gastrodermis. Between these two layers is a binding gluelike layer of material known as the mesogloea or coenenchyme. The tentacles have some specialized stinging capsules, known as nematocysts. These capsules can be “fired,” turning rapidly inside out to release a whiplike thread that is often tipped with poisonous spikes.

Since they are very simple animals, corals lack a brain. But they have muscles and a very simple nervous system that enable them to move their tentacles to capture food and also to retract into the body if there is a threat. Inside the mouth is a throatlike pharnyx and then a wide body cavity, the coelenteron. In most corals this coelenteron has long vertical folds of body tissue reaching toward the center. The vertical folds are known as mesenteries.

Generally speaking coral polyps do not move about. Many are fixed by a solid skeleton to the seafloor. The types of skeleton used by corals are highly varied.