What is the reason for the name of the type of coelenterates?

The name is associated with the internal structure of the body of these animals. Food enters through the mouth into the intestinal (gastric) cavity, where it is digested and absorbed, and digestive waste is removed in the same way – through the mouth. This digestive system is called closed.
Digestion occurs not only in the gastric cavity (cavity digestion), but also in the cells lining it (intracellular digestion). For coelenterates, the presence of stinging cells and the alternation of the phases of development of a floating jellyfish and a sitting polyp are characteristic. There are three classes of coelenterates: Hydroid, Scyphoid, Coral polyps.