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Also known as the “cannonball jellies” because the rounded bells of some species (e.g. Stomolophus meleagris) resemble an old‐fashioned cannonball (dangling an old‐fashioned petticoat: Figure 3.12c), the rhizostome medusae lack marginal tentacles and exhibit a proliferation of the oral arms (Figure 3.13). During early development, the lobes of the original four‐lobed mouth in the very young medusa grow and bifurcate (Figure 3.13a–c) to form the (usually eight) “mouth arms” typical of the group. As the oral arms develop, the fringed edges close over to form an inner tube or brachial canal that effectively eliminates the central mouth. Once the canal is closed over, the fringed edge forms an outer groove that communicates at intervals with the inner brachial canal through smaller canals that terminate in “suctorial mouths.” Most often the oral arms are invested with filamentous appendages that bear the suctorial mouths along with nematocyst batteries and mucous cells that aid in prey capture (Hyman 1940) (Figure 3.13d–f). The brachial canals lead to a central stomach that conveys nutrition to the bell via radial canals.

Figure 3.13 Rhizostomeae. Development of the mouth arms of Mastigias: (a) early stage, (b) lobe development, and (c) later stage. (d) Mouth arm of Cassiopeia showing the gastrovascular canals; (e) adult Cassiopeia. (f) Adult Rhizostoma pulmo.

Sources: (a) Adapted from Uchida (1926); (b and c) Kaestner (1967), figure 5‐17 (p. 105); (d and e) Hyman (1940), figure 172 (p. 525); (f) Redrawn from Mayer (1910), plate 73.

The life histories of rhizostome medusae are typical of the Scyphozoa (Figure 3.12c) in having a planula larva that settles to the bottom and forms a polypoid scyphistoma. Scyphistomae may strobilate to form ephyrae or may produce other scyphistomae by budding.

The rhizostomes are chiefly a tropical–subtropical group inhabiting shallow waters, though two genera, Rhizostoma and Stomolophus, are found in temperate climes and may even form blooms. Stomolophus nomurai forms huge blooms in the Sea of Japan at intervals (Mills 2001). Rhizostomes are generally quite large, with sizes ranging from 4 to 200 cm across the bell.

Genera include: Rhizostoma, Mastigias, Cassiopeia, Stomolophus, Cephea.