IT has been known since 19111 that the blood of certain species of ascidians contains organically bound vanadium. Bertrand2 has shown that most marine invertebrates contain between 0.2 and 2 parts per million of vanadium dry weight, whereas in ascidians the figure may be as high as 6,500 p.p.m. dry weight. This high level is not, however, true of all species, and in some families the level, where investigated, is not significantly above that found in other groups of marine invertebrates. Didemnum candidum, for example, is quoted as containing 2.6 p.p.m.