• 1) Well defined phylum of ~10,000, mostly marine, predatory species divided into three main classes (Hydrozoa, Scyphozoa, Anthozoa)

• 2) Two body forms predominate: polyp (benthic) & medusae (pelagic), alternation of generations is widespread

• 3) The basic body plan of all cnidarians is similar:

  • a) radially symmetrical
  • b) the gut has one opening (mouth only, no anus)
  • c) hollow (usually) tentacles surround the end bearing the mouth; tentacles capture prey and transport food to the mouth
  • d) the gastric cavity has associated canals (=gastrovascular cavity)
  • e) the body wall has three, well-defined layers

• 4) Cnidarians exhibit a true tissue level of organization (groups of differentiated cells working together for a single function)

  • a) ectodermis (outer) & endodermis (inner) are true tissues
  • b) the middle layer (mesoglea) is mostly non-cellular & not a true tissue layer, so cnidarians are diploblastic

• 5) This diverse phylum is united by a unique and complex cell type:

  • a) a cnidocyte is a cell that bears two important organelles:
    • nematocyst (capsule with an inverted coiled tube & operculum)
    • cnidocil (sensory cilium= "hair trigger")

• 6) Nematocysts come in many forms (grappling, penetrating, sticky); one to several forms may occur in a single individual; they aid in feeding (mainly), and also in defense & aggression

• 7) Are only a few other cell types; most occur in ecto- & endoderm:

  • a) epithilio-muscular cells (cells with contractile myofibrils in base)
  • b) interstitial cells (totipotent cells; produce gametes & cnidocytes)
  • c) receptor cells (often have sensory cilium)
  • d) nerve cells (form loosely organized net); no contractile elements
  • e) gland cells (mainly in endodermis); no contractile elements
  • f) amoeboid cells (mobile cells in mesoglea; produced by ectoderm)

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