1 Shell or test.
2 The cell wall in some protistan algae, e.g. diatoms and dinoflagellates (see dinophyceae).
3 The dense wall at the margin of a corallite in a scleractinian coral (Scleractinia).
4 In graptolites (Graptolithina), one of the cup-like structures that occur along the stipe, each of which houses an individual member of the colony. Thecae vary from simple, straight-sided tubular structures to those with an introtorted opening, where the thecal aperture faces downward. There are triangulate thecae with very small apertures and in some species the thecae are long, thin, and separated from one another (‘isolate’). In some monograptids the thecae vary in structure along the length of an individual stipe as well as from one species to another. Thecal shape also varies through time.