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The blood of a number of species of Ascidiacea has been examined with reference to the presence of the vanadium chromogen described by Henze, and the results are discussed in connexion with the data available from other authors. The chromogen is always contained, together with sulphuric acid, in a special type of cell described as a vanadocyte. These are found only in the hcidiidae and Perophoridae, though members of certain other families contain vanadium in some other form. The distribution within the group of sulphuric acid in vesicular cells of the blood and test is also discussed. There is no reason to suppose that the pigments of Ascidians, other than the chromogen itself, are vanadium compounds. A curious type of cell inclusion from the orange pigment cells of Ascidia mentula is described. Experimental evidence and theoretical considerations are brought forward to show that the vanadium chromogen is not in any sense a respiratory pigment. At present no function can be ascribed to it. I t is not a protein or porphyrin compound, but seems to consist of vanadium in association with a straight chain complex of pyrrol rings, comparable perhaps to a bile pigment. It seems possible that sea water provides a sufficiently rich source of vanadium to account for the amounts absorbed. The phylogenetic aspects of these facts are considered in relation to Berrill's recentre-classification of the Ascidiacea. The presence of vanadium is a primitive character which has been lost in the more specialized families.

Observations On the Blood of Certain Ascidians, with Special Reference to the Biochemistry of Vanadium