- Members of the class Hydrozoa are medusoid or polypoid or exhibit both forms in their life cycle. The mesoglea is acellular, cnidocytes are restricted to the epidermis, and gametes develop in the epidermis. Hydrozoans may be the most primitive of the three classes of cnidarians.
- Hydromedusae are usually small and planktonic.
- The most primitive hydrozoans are probably medusoid species, in which the pelagic actinula develops directly into an adult medusa. Such a life cycle may also be primitive for the phylum.
- The polypoid form may have arisen in some medusoid species in which the actinula passed through a period of attachment prior to development into a pelagic adult; i.e., the attached actinula was the first polyp.
- Early polypoid stages, including the attached actinula, probably reproduced asexually by bud¬ding. Persistent attachment of the buds led to colonial polypoid species, called hydroids, which now compose the majority of hydrozoans.
- Associated with colonial organization has been the evolution of a skeleton (support) and polymorphism (division of labor).
- Naked solitary species, such as hydras and the Gonionemus polyp, probably stem from early polypoid forms that were not colonial.
- Suppression of the medusa through attachment to the polyp and subsequent reduction has evolved independently in different hydrozoan lines, and living species exhibit all degrees of reduction in the medusoid form.
Class Scyphozoa-Scyphozoa. Coral polyps class-Antozoa.
Scyphozoans are the cnidarians most frequently referred to as jellyfish. In this class the medusa is the dominant and conspicuous individual in the life cycle; the polypoid form is restricted to a small larval stage. In addition, scyphozoan medusae are generally larger than hydromedusae. The majority of scyphozoan medusae have a bell diameter ranging from 2 to 40 cm; some spe-cies are even larger.