The gastrovascular cavity varies in shape and size among the scyphomedusae. It consists of an usually four-sided stomach and peripheral extensions, sometimes as a series of pouches or canals, which form a circulatory system open to the stomach cavity. The peripheral regions, pouches, or canals allow food to be distributed from the stomach cavity around the bell, to tentacles, and to the oral arms in rhizostomes. Spatially or temporally separated currents, effected by ciliary motion, allow waste products to be transported back to the stomach and then the mouth where they are expelled.

In semaeostomes and rhizostomes, the distribution of pouches and canals varies considerably and, like other morphological characters, has been used to distinguish species. For example, Stiasny (1921) stained and drew the canal systems of rhizostomes in order to determine their relationships. Similarly, Uchida (1926) proposed phylogenetic relationships among the semaeostomes and rhizostomes based on the structure of the canal system (see here). Greenberg et al. (1996) found that canal structure was one of the few morphological features that could be used to distinguish moon jellyfish that differed genetically. However, like other morphological features of jellyfishes, the form of the canals can be highly variable and may not always be a reliable indicator of species boundaries or relationships (e.g. Dawson 2003).


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