Zoology 250 Lecture 9
PHYLUM CNIDARIA (II): Diversity
- 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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