\The"ca\, n.; pl. {Thec[ae]}. [L., fr. Gr. ? a case to put
anything in. See {Tick} a cover.]
1. A sheath; a case; as, the theca, or cell, of an anther;
   the theca, or spore case, of a fungus; the theca of the
   spinal cord.
2. (Zo["o]l.)
   (a) The chitinous cup which protects the hydranths of
       certain hydroids.
   (b) The more or less cuplike calicle of a coral.
   (c) The wall forming a calicle of a coral.