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There exist four respiratory pigments capable of uniting with oxygen in such a manner that the oxygen is given up to a vacuum. Of these four, hæmoglobin has a wide distribution in verterbrate and invertebrate animals (Redfield, 1933), chlorocruorin is found only in certain polychaete worms (Fox, 1926, 1932, 1933), hæmerythrin in sipunculid worms (Florkin, 1933) and hæmocyanin in arthropods and molluscs (Redfield, 1934). The first three pigments named contain iron in the molecule, while the last has copper. Since the work of Peters (1912) it has been known that in oxyhæmocyanin Dhéré (1916, 1919) first showed the approximate proportionality between the oxygen and copper contents, and later Begemann (1924), Redfield, Coolidge, and Montgomery (1928) and Guillemet and Gosselin (1932) established the ratio of one molecule of labile oxygen to two atoms of copper. For oxyhæmerythrin it has recently been shown by Florkin (1933) that one molecule of oxygen is united to three atoms of iron. Up to the present, however, the ratio of oxygen to iron in oxychlorocruorin has not been known, and the work reported below was undertaken to settle this question. The quantity of iron present in the blood ofspirographis spallanzanill , both combined with chlorocruorin and free, has been analysed, and, the total oxygen capacity of the blood being known, the oxygen to iron ratio of oxychlorocruorin has been determined. For iron analysis pure undiluted blood was extracted from the crown ofSpirographis at the Tamaris Marine Biological Station. The technique already described (Roche and Fox, 1933) was used for obtaining the blood, with this difference, that the crown was amputated by means of a glass cover-slip so that no iron instrument came into contact with the blood. Eight measured specimens (0.2 cc each) of blood, each derived from several individual worms, were diluted with distilled water and sealed in test tubes.

The oxygen to iron ratio of oxychlorocruorin and the total quantity of oxygen carried by the pigment in spirographis