1. (n.) 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. (n.) The chitinous cup which protects the hydranths of certain hydroids.

3. (n.) The more or less cuplike calicle of a coral.

4. (n.) The wall forming a calicle of a coral.