Marine Science 2016 Edition

Page 276 Chapter 11 Figure 276.1 Cnidarians are radially symmetrical. (Illustration Bob Moffatt) Figure 276.2 Animals can be either free swimming or attached. Phylum Cnidaria Sac -like animals with body plan arranged in a circle and a sac at one end, two cell layers, stinging cells. Includes animals such as corals and sea jellies. External features These animals are radially symmetrical, with a simple body in the form of a sac or coelenteron with one opening or mouth as shown in Figure 276.1. Animals can be either attached by means of a basal disc to form a polyp (eg corals ) or may drift in the water as a free swimming medusa such as sea jelly (Figure 276.2). Medusae have a bell with balance organs known as statocysts and tentacles located below the bell Figure 276.2. Attached Statocysts Tentacles Free swimming (Steven Byers) (Sue Oats) Bob Moffatt (Sue Oats) Bell