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• 1) Odd phylum of ~100 species; all marine, most pelagic; can be key predators in mid-ocean environment (eat fish larvae & zooplankton)

• 2) Three distinctive features:

  • a) possess eight vertical "comb rows"; combs are formed from groups of cilia; include largest cilia known in Metazoa

  • b) two long, branching tentacles covered with colloblasts (unique, very adhesive cell type) extend from deep tentacle sheaths

  • c) exhibit conspicuous biradial symmetry

• 3) Share three features with Cnidaria:

  • a) diploblastic organization with extensive middle layer (mesoglea?)

  • b) possess only a mouth (no true anus)

  • c) gastrovascular cavity with radiating canals

• 4) Exhibit some curious differences from Cnidaria:

  • a) asexual reproduction not well-developed

  • b) all are hermaphroditic

  • c) highly determinate, bi-radial cleavage

  • d) lack cnidocytes

• 5) Evolutionarily enigmatic; some believe related to deuterostomes

  • a) middle layer exhibits tissue-like characters (muscle cells, nerve cells, mesenchyme cells)

  • b) middle layer develops from mesenchyme cells in a way that resembles primary mesenchyme formation in echinoderms

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