Online Biology Dictionary


A nematocyst (also known as a cnidocyst) is a capsule or sac within a cnidocyte, the stinging cell of a cnidarian. Its action is much like that of a poisoned harpoon.

The diagram above shows the sequence of events that occurs during a sting:

  • At left is an as yet un-triggered cell (yellow rectangle) with its oval nematocyst, which contains the entangling threads and barbed, venomous projectiles fired by the cnidocyte. When the cnidocil trigger is touched, the cnidocyte is stimulated. This causes a covering flap—the operculum—to fly open.
  • Middle image: The open operculum, the rapidly uncoiling thread and the emerging barb, which are fired with extreme rapidity.
  • Far right: The fully extended cell.

The venomous barbs enter the victim and the threads entangle it, which allows the cnidarian to keep its prey in close contact to administer additional stings and eventually ingest it.

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