Nematocyst

Often one sees, on surface view, a distinctly rounded vacuole, occupying a position in the cell occupied in others by a large nematocyst.

The young nematocyst decreases in size and becomes coarsely granular, the granules arranging themselves in a spiral line close under the external membrane.

Nematocyst discharge is another example of the Rubicon principle – it either fires or it does not.