Sabella pavonina Savigny lives in a narrow cylindrical tube which it irrigates by means of piston-swellings travelling tailwards along the body. Decapitated worms can live and regenerate new heads in glass tubes open at both ends, which they irrigate in the same way as complete worms. If tubeless worms are bisected at the junction of thorax and abdomen, the total oxygen consumption is unaffected by the operation. These results suggest that the crown surface is of no importance in the respiratory supply of the hinder part of the body as long as the surface of the latter has free access to oxygenated water. If, however, worms are put in tubes whose lower ends are closed so that through irrigation is impossible, crowned worms can live but crownless worms soon die or leave the tubes. This shows that the crown can supply the hinder parts when irrigation is prevented. Myxicola infundibulum Renier lives in a gelatinous tube. There is no irrigation current, either through the tube or in and out at the head end. Put in a glass tube, the worm fills most of the lumen, and plugs the hinder end, with jelly. The worm can live for many weeks in a tube sealed at the lower end. The respiratory exchanges of the whole worm therefore appear to take place through the anterior end, and especially the crown. Bisection of tubeless worms at the junction of thorax and abdomen results in a decrease of the total oxygen consumption to about two-thirds of its previous value; this is attributed to the separation of the abdomen from its usual respiratory surface. Owing to structural differences, the velocity of the water current through the crown is considerably greater in Myxicola infundibulum than in Sabella pavonina. The frequency of pulsation of the crown vessels is, however, about the same, and the differences in crown structure are probably food-collecting rather than respiratory specializations. The ability to survive and regenerate after injury to the anterior end is very much less in Myxicola infundibulum than in Sabella pavonina. This fact is discussed in relation to the respiratory and other differences between the worms.