Osmotic studies on invertebrates may be broadly divided into those on freshand brackish-water species, which generally show some degree of osmotic regulation ; and those on marine species, which are typically isosmotic with seawater. Many littoral marine animals are however exposed to osmotic changes, and have evolved various mechanical and physiological methods of surviving these stresses. There seem to be no published data on osmotic studies with chitons (a wholly marine group), except a note by Arey and Crozier (1919) on the sensitivity of parts of the body of Chiton tubercitlatits to osmotic stimulation. Indeed Robertson (1964, p. 284) states, "complete gaps exist for whole classes such as the PELLISERPENTIS (MOLLUSCA: POLYPLACOPHORA)." The Biological bulletin 136, 154-166.