Thetis was a Greek goddess of rivers and oceans, one of the daughters of Nereus, who took responsibility with Okeanos for these bodies of water. She was a minor deity who, according to mythology was a mermaid. She was of particular significance as the mother of Achilles, fathered by an unnamed mortal. According to legend, her attempt to make Achilles mortal failed because the heel by which she held him by when immerging him in the river Styxremained dry. She entrusted his education to the centaur Chiron. Nereides, attendant sea creatures, surrounded her, and after Achilles' death she returned to the ocean depths. A.G.H.
Jordan, Michael, Encyclopedia of Gods, New York, Facts On File, Inc. 1993, p. 259