From ZooTerms (Dictionary of Invertebrate Zoology)
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theca (noun; plural thecae; Greek theke, case): 1. A sheath or sac-like covering or structure for an organ or organisms, as proboscis, tubes, shells, pupa or larvae.

2. (Annelida: Oligochaeta) Spermatheca.

3. (Arthropoda: Insecta) A fold or sheath from phallobase enclosing the aedeagus.

4. (Echinodermata: Crinoidea) The skeleton.