Theca

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.
1. The chitinous cup which protects the hydranths of certain hydroids.
2. The more or less cuplike calicle of a coral.
3. The wall forming a calicle of a coral.