theca
From ZooTerms (Dictionary of Invertebrate Zoology)
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.
