- Members of the class Scyphozoa are pelagic cnidarians in which the medusa is the dominant and conspicuous form. A polypoid larva, equivalent to an actinula, follows the planula. Assuming the primitive nature of the medusoid form, scyphozoans are primitive in their life cycle and perhaps evolved early from the ancestral hydrozoans.
- Within the Scyphozoa, specialization has led to complexities in medusoid structure, as evidenced by such features as the following: larger size than that of most hydromedusae, more highly developed manubrium, cellular mesoglea, septate gut or at least a gut with gastric filaments, gastrodermal cnidocytes, and some development of sense organs.
- The gonads are gastrodermal, and the eggs, which are shed through the mouth, develop into planula larvae. Following settling, the planulae develop into polypoid larvae, which feed and may reproduce asexually.
- In some species the polypoid larva transforms directly into a young medusa, which can be taken as additional evidence that the polypoid form was derived from a larval stage in the evolution of the cnidarians. In most species of scyphozoans, young medusae are budded off transversely from the oral end of the polypoid larva.