1) Alternation of generations (polyp <-> medusa) and sexual reproduction are widespread; fertilization is mainly external; radial cleavage is followed by a blastula & gastrula stage
a) a distinctive ciliated planula larva forms after gastrulation
b) the planula larva may be the ancestral form of the Bilateria
2) Cl. Hydrozoa (hydroids & relatives) exhibit a bewildering array of life-cycles; some emphasize the polyp and others the medusa stage:
a) Hydroids- solitary or colonial polyp form predominates
polyps thecate (e.g., Obelia) or naked (e.g., Hydra); distinguishes two orders
division of labor among polyps in colonial forms
most still possess a medusa stage for sexual reproduction
b) Trachyline medusae- polyp stage reduced or absent
c) Hydrocorals- polymorphic polyp colonies with massive carbonate skeletons; medusa stage reduced
d) Or. Siphonophora- swimming or floating polymorphic colonies; may have multiple medusoid & polypoid forms in one colony!