The cirratulid polychaete Cirriformia tentaculata (Montagu) may be found in gravelly mud or sand often beneath large stones in pools between tide marks. It is usually associated with reducing conditions indicated by blackened mud or sand and the distinctive odour of hydrogen sulphide. It occurs also in crevices that retain stagnant water at low tide. The worm does not make a permanent burrow but lies in different postures to enable the branchiae and feeding tentacles to be extended on to the surface, where they are bathed by the overlying water. The mucus-lined burrow is not irrigated.

Cirriformia tentaculata encounters two respiratory problems. When covered by water the branchial filaments can take up oxygen which is then circulated by means of the well-developed vascular system with blood containing haemoglobin in solution. While the branchiae may lie in a well-oxygenated medium the main body of the worm lies in a highly reducing substrate. The sediment immediately surrounding the worm is commonly black and there is no oxygenated layer as occurs in burrows which are irrigated and provide an envelope separating the animal from a potential oxygen-sink. If exposed by the tide the branchiae are withdrawn or clump together and cannot function effectively. Even when covered by the tide the branchiae can be rapidly with-drawn, presumably to avoid loss by predation, and under all these conditions the animal must experience hypoxic or anoxic conditions of varying duration of up to several hours.

We have examined the ability of Cirriformia tentaculata to maintain oxygen uptake under decreasing oxygen tension, the oxygenational properties of the haemoglobin, the oxygen capacity and blood volume.