ABSTRACT
It has been shown that the oxygen consumptions of various anthozoans are generally comparable with those of other invertebrates if the dry weights rather than the wet weights are considered (Brafield & Chapman, 1965). We have also suggested that the endoderm is probably the major site of oxygen uptake in pennatulids, associated with water movements into and out of the enteron. Three main sources of evidence support this view. First, the enteric water has a low oxygen concentration. Secondly, a fully contracted specimen consumes oxygen at a low rate, possibly due to a severely limited enteric circulation. Thirdly, measurements of the oxygen concentration of the water around a specimen in a closed vessel are sometimes inconsistent. Such variations might well be partly due to fairly regular movements of water in and out of the enteron, the inconsistent estimations having been made either just before or after an efflux of relatively deoxygenated enteric water. This source of inaccuracy in measuring consumptions can be avoided only if the oxygen concentration is measured continuously. Consequently experiments have been conducted in which a continuous-flow respirometer was used in an attempt to demonstrate whether a rhythmic enteric irrigation does occur in pennatulids; for if it does, and water which has become partly deoxygenated by oxygen uptake through the endoderm is periodically expelled from the enteron, a continuous record of the oxygen concentration of water which has been flowing slowly past the animal should show a rhythmic rise and fall. Pteroides griseum was used in these experiments because this species has a spacious enteron, and because a good supply of large specimens was available.
