Water lung and body wall contributions to respiration in an echiuran worm.

The modified hindgut of the echiuran marine worm Urechis catq~o functions as a water lung and has been assumed to be a much more important respiratory surface than its thick, muscular body ,a, all. We tested this assumption by measuring whole animal 02 consumption, hindgut O_, uptake, and hindgut ventilation activity simultaneously in unrestrained worms in artificial burrows from 25 to 300 "Ion- 02. Under experimental conditions the contribution of the hindgut to total 02 uptake is variable and strongly correlated to hindgut ventilatory activity. Over a Po, range simulating that encountered in the natural environment, the hindgut accounts for approximately halt of total O: uptake on average. Under progressive hypoxic exposure total Oz consumption decreased by 50%, yet O2 conductance and extraction increased. The results suggest that the water lung function of the modified hindgut supplements 02 uptake across the body wall, and ma) bc especially im~wtant during periods of high activity such as may occur during feeding and burrowing. Kevw,~l,w (;as exchange, water, hindgut (echiuran marine worm)- Invertebrate. echiuran marine worm Urechis caupo: Lung, water