the·ca 
(thēkə)
(thēkə)n. pl. the·cae (-sē′, -kē′)
A case, covering, or sheath, such as the covering of the pollen sacs of an anther or the thick cell wall of certain dinoflagellates.
[Latin thēca, case, receptacle, from Greek thēkē; see dhē- in the Appendix of Indo-European roots.]
thecal (-kəl) adj.