Tē'thys, n.

n. [Gr. τῆθος or τῆθους, an oyster, or a kind of ascidian; Lat. tethea, a kind of sponge; Fr. téthye.] (Zoöl.) A gelatinous, nudibranchiate, gastropodous animal, having an envelope or mantle that extends above and beyond the head, fringed or undulated at the margin.