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Classification within the Cnidaria is constantly evolving. While there is consensus on what the different groups within the Cnidaria are, there is considerably less agreement on rank and position within the systematic hierarchy. It is important to recognize that molecular methodologies have opened a new and rapidly evolving way of classifying species at all levels of the taxonomic hierarchy. The half‐life of any taxonomic scheme will likely be quite short for some time to come, and the only way to keep up will be with web‐based systems such as the world register of marine species (WoRMS) or with taxon‐specific sites. The scheme shown below is that of Brusca et al. (2016). Brusca et al. divide the Cnidaria into three subphyla: the Anthozoa, Medusozoa (cf. Bouillon 1999), and the parasitic Myxozoa. The Anthozoa are devoid of the medusa phase, and in the Medusozoa the medusa phase is an important phase of the life cycle. The Anthozoa include one class, the Anthozoa, and two subclasses: the Hexacorallia and Octocorallia. All are benthic forms, with the anemones, black corals, and stony corals included in the subclass Hexacorallia, and the gorgonians, soft corals, sea pens and sea pansies and organ pipe corals included in the subclass Octocorallia.

The Medusozoa include the taxa of main concern, the free‐swimming pelagic jellies. Five classes of Medusozoa have been identified, of which three, the Cubozoa (cubomedusae), Scyphozoa (the large jellyfishes), and the Hydrozoa (hydroids and hydromedusae), will garner the most attention in the following pages. Though anthozoans are major players in the benthos of all oceanic systems, our chief interest here is their structural relatedness to the free‐swimming jellies. The similarities between polyps and medusae are best visualized by turning a medusa upside down (Figure 3.1). It is easy to see that both polyps and medusae may be characterized as having a central gut (a gastrovascular cavity or stomodeum) surrounded by a row of radially arranged tentacles.

Figure 3.1 Tissue layer homologies in cnidarians. (a) A hydrozoan polyp; (b) an anthozoan polyp; (c) a hydromedusan medusa, shown upside down for similar orientation. The outer tissue layer is ectodermal (epidermis); the inner tissue layer is entodermal (gastrodermis); the middle tissue layer is mesenchyme/mesoglea.