Sinh học - Chapter 42: Circulation and gas exchange

The body plan of a hydra and other cnidarians makes a circulatory system unnecessary. A body wall only two cells thick encloses a central gastrovascular cavity that serves for both digestion and for diffusion of substances throughout the body. The fluid inside the cavity is continuous with the water outside through a single opening, the mouth. Thus, both the inner and outer tissue layers are bathed in fluid.