• 1) Life cycle: alternation of generations (polyp <-> medusa) and asexual 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 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!

• 3) Cl. Scyphozoa (jellyfish), medusa form prominent

  • a) medusae possess: large complex manubrium, gastrodermal cnidocytes, and notched bell margin with rhopalia
  • b) new medusae are produced from a polyp by strobilation
  • c) some polyp stages possess septae in the gastrovascular cavity

• 4) Cl. Anthozoa (anemones, corals), medusa form absent

  • a) polyps possess: septae in the gastrovascular cavity, gastrodermal cnidocytes, tubular pharynx with siphonoglyphs

  • b) Octocorals- diverse colonial forms (soft corals, gorgonians & sea pens) typically having 8 pinnate tentacles & 8 septae

    • tanned collagen skeletons of gorgonians are endoskeletons

  • c) Hexacorals- heterogeneous group (solitary anemones & stony corals); more than 8 tentacles & septae (typically multiples of 12)

    • massive carbonate skeletons of corals are exoskeletons

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