The phylum Cnidaria is comprised of three major clades: Anthozoa (which includes the hard corals, soft corals, sea anemones, and tube anemones), Medusozoa (which includes hydroids, true jellies, box jellies, and stalked jellies) and Endocnidozoa (which are endoparasitic cnidarians!!). Although they may appear simple, morphological diversity varies immensely across Cnidaria. Tube anemones (Ceriantheria) get their name from the fact that they live in - wait for it! - a tube. ("Why is that so special?", asked the tube worm.) These tube-dwelling anemones are particularly fascinating to us because their tube is constructed from the discharged contents of a specialized type of cnidocyte found only in tube anemones: the ptychocyte (pronounced "tick-o-cyte"). Ptychocytes lack a piercing apparatus and are not known to harbor venom, but are these ancestral or derived traits? The phylogenetic position of Ceriantheria - sister to the hexacorals - makes this group a particularly valuable model for understanding how cnidocyte form and function diversified after the origin of this novel cell lineage in the stem cnidarian.
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