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Swimming in medusae is a rhythmic process that is controlled by neural pacemakers and communicated to the swimming musculature via a neural network known as a nerve net. Nerve nets in the cnidaria are at their most basic in polyps and comprise two grids, one net beneath the outer epidermal layer and one beneath the inner gastrodermal layer. Both nets are located between the outer epithelium and the mesoglea and are considered literally to be a diffuse network, with no polarity in signal propagation. That is, signals propagate equally in all directions.

In the hydromedusae, the inner and outer nerve nets have partially been consolidated into marginal nerve rings located at the inside (subumbrellar) and outside (exumbrellar) of the umbrellar margin (Figure 3.22). The inner nerve ring communicates with the swimming musculature as well as with the marginal sense organs and tentacles and governs the rhythm of the swimming musculature.

With the exception of the coronates, the scyphomedusae do not have the well‐defined marginal neural rings of the hydromedusae. Nonetheless most of the action in their nervous system takes place at the umbrellar margin because that is where the sense organs and tentacles are located and, as in the hydromedusae, it is also where the swimming rhythm is generated. The neural networks are a bit more complicated in the scyphomedusae and considerably more is known about them.

The nervous system of scyphomedusae is composed of three parts: the motor nerve net, the diffuse nerve net, and the marginal centers (Arai 1997). The motor nerve net innervates the swimming muscle, the diffuse nerve net conveys sensory information to the marginal centers, and the marginal centers act as pacemakers for the swimming rhythm and integrators for the sensory information provided by the sensory apparatus. Even though only one of the nerve nets is termed “diffuse,” both nerve nets are highly complex and diffuse networks that intercalate through different tissues. The neural tissue is difficult to isolate and even more difficult to map. In fact, the two nerve nets are mainly defined by their function, which was described using physiological methods (Anderson and Schwab 1981).

Figure 3.22 Nerve net in a hydromedusa.

Source: Kaestner (1967), figure 4‐20 (p. 61). Reproduced with the permission of John Wiley & Sons.

The marginal centers are important junctions for integrating sensory inputs and conveying them as needed to the swimming muscle. They are located directly behind the rhopalia on the bell margin, most easily visualized in the coronates (Figure 3.23a). Their precise location is unknown, but the fact that the centers are located very close to, but not within, the rhopalia itself was determined by methodical and highly localized ablation experiments in the 1980s (Passano 1982). The criterion for determining the location of the marginal center was the presence or absence of a pacemaker signal to the swimming musculature.