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The Cnidarians are all radially symmetrical (Figure 3.2); their parts are arranged around a central axis much like the spokes of a bicycle wheel are arranged around the hub. Each of the radii has names that are used in classification. The central axis or hub is the oral–aboral axis or gut. Between the “hub” and the “rim,” body parts are arranged concentrically (Figures 3.3 and 3.4). In perfect radial symmetry, a plane bisecting the animal in the axis of the hub will result in two perfectly equal halves, no matter where the plane is placed. Most often, and even in the hydromedusae, the morphology is modified to biradial (two equal halves) or tetramerous (four equal quarters) radial symmetry. The radial symmetry of medusae is important to their foraging strategy. Prey are equally accessible from all points of the compass, which, for an ambush predator, is a distinct advantage.

Figure 3.3 shows the basic body form of a hydromedusa. The outer, or exumbrellar, surface and the inner, subumbrellar, surface are separated by a deformable gelatinous acellular mesoglea. The mesoglea acts as a primitive skeleton, giving a medusa its characteristic shape and providing a surface against which muscle tissue can act to propel the medusa through the water.

Figure 3.2 Diagram to define the radii of a medusa. P. are the per‐radii on which the four primary radial canals lie. I. are inter‐radii and A. are ad‐radii.

Source: Adapted from Russell (1954), figure 1 (p. 2).

Figure 3.3 Diagram of a medusa with one quadrant removed.

Source: Adapted from Russell (1954), figure 2 (p. 3).

Swimming bells range in shape from tall to spherical to highly flattened in appearance (Figure 3.5a, Russell 1954). The stomach is a simple sack that projects downward from the subumbrellar surface and can vary considerably in length and morphology. The opening of the stomach or “lips” may vary from a simple circular opening to a highly crenulated and folded appearance. In some cases, the mouth is surrounded by small oral tentacles (Figure 3.5b).

Figure 3.4 Cnidarian structure. (a) Diagram of a hydroid polyp; (b) cross section of a hydroid polyp; (c) diagram of a hydrozoan medusa; (d) cross section of a hydrozoan medusa.

Source: Adapted from Hyman (1940), figure 106 (p. 368)

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The radial canals originate at the four corners of the stomach and extend along the subumbrellar surface to join the ring canal that runs along the umbrellar margin. The umbrellar margin contains the marginal tentacles that are primarily used for subduing prey. The stomach, radial canal, and ring canal system provide for the distribution of nutrients to the medusa as a whole.

Figure 3.5 Medusae structure. (a) Diagrams of umbrella shapes of different medusae; (b) diagrams of mouths of different medusae.

Sources: (a) Russell (1954), figure 3 (p. 4); (b) Russell (1954), figure 55 (p. 5).