1. Littoral representatives of the genus Nephthys are described as burrowing forms, able to irrigate their burrows with well-aerated sea water except when the sand is exposed by the tide. Then they are sealed in and have no access to oxygen outside the sand.
2. The concentration of dissolved oxygen in the sand water corresponds to a tension of about 7 mm. of Hg compared with a value about twice as great in the residual water in Arenicola burrows.
3. Extracellular pigments in the blood and coelomic fluid of Nephthys spp. are characterized as haemoglobins. The quantity of these pigments is shown to be inadequate as an oxygen store for the exposure period.
4. Dissociation curves for both pigments from N. hombergii are found to be approximately hyperbolic and the oxygen affinities relatively low.
5. The significance of the difference in oxygen affinity and the direction of the Bohr effect, between the vascular and coelomic pigments cannot yet be evaluated.
6. It is suggested that the Nephthys pigments are unspecialized and may function as a high-tension oxygen transport system only when the sand is covered by the sea. This is contrasted with the possibility of low-tension oxygen transport by the haemoglobin of Arenicola.