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The semaeostomes include the medusae most typical of the class and most familiar to beachgoers. Medusae are large, typically 5–40 cm, with a bell shape ranging from saucer‐like (Aurelia) to bowl‐like (Chrysaora) and lack a coronal groove. Tentacles are found along or below the margin of the umbrella, which may be divided into eight or more lappets. Most typical of the semaeostomes are the long frilly oral arms that originate at and form the corners of the mouth (Figure 3.10).

Life histories for many of the semaeostomes are known in detail, owing mainly to their abundance near shore. Two life histories have been noted. The sequence of the first (Figure 3.12a), typified by the moon jelly Aurelia, has a planula larva that develops from the fertilized egg and settles to the bottom to form a scyphistoma. The scyphistoma strobilates to form ephyrae that develop into either a mature male or female medusa. In the second life history strategy, seen in the genus Pelagia, the planula develops directly into an ephyra and then into a mature medusa, bypassing the benthic polyp, or scyphistoma, stage (Figure 3.12b).

Figure 3.12 Life cycles of scyphozoans. (a) Aurelia sp. (moon jellyfish); (b) Pelagia sp. (purple‐striped jellyfish). (c) Stomolophus meleagris (cannonball jellyfish).

Sources: (a) Bayer and Owre (1968), figure 161 (p. 105); (b) Bayer and Owre (1968), figure 162 (p. 106); (c) Adapted from Calder (1982), figure 4 (p. 156).

Genera include Aurelia, Chrysaora, Cyanea, Stygiomedusa, and Pelagia. Pelagia, as the name implies, is common in offshore waters