Fig. 2. (A) Dorsal view of the microcrustacean Daphnia magna showing the medium flow pattern (white arrows) and the circulatory pattern (black arrows). A dorsal piece of the left carapace valve (chequered area) was removed (for details see Pirow et al., 1999b). (B) Conceptual model for oxygen transport in D. magna based on a cylinder-within-a-tube arrangement. Medium flows through the space between the carapace and the trunk in a posterior direction (open arrows) while oxygen is released both into the carapace lacuna and the peripheral tissue layer of the trunk. This tissue layer is supplied with oxygen from the medium and from a truncal haemolymph space by diffusion (broken arrows). Oxygenated haemolymph leaves the double-walled carapace and then enters the truncal haemolymph space (solid arrows). While flowing in an anterior direction, oxygen diffuses from this haemolymph space both into the coaxial tissue cylinder and the cortical tissue layer (broken arrows). P_in, P_ex, inspiratory and expiratory oxygen partial pressures, respectively; P_a and P_v, oxygen partial pressures of the haemolymph entering and leaving the trunk, respectively.