Before fixation, older Helobdella embryos (stages 10 & 11) and juveniles requires anesthesia so the specimen does not become twisted resulting from the animal's reflex response to the fixative. To relax the embryos or juveniles, bath them in ice-cold "relaxant" for a few minutes. Stage-11 embryos and juveniles should become "straightened" before they are ready to be fixed. The recipe for the relaxation buffer is 4.8 mM NaCl, 1.2 mM KCl, 10 mM MgCl₂, 8% ethanol in ddH₂O. The effect of relaxant is reversible. Do not return relaxed embryos and juveniles into regular culture medium if you are going to fix the embryos or juveniles.