1. If a sponge is exposed to moving water, it is physically possible for water to pass unidirectionally through it without active pumping by the choanocytes; mechanisms presented for such "passive flow" predict flow from ostia to oscula. 2. In actively pumping Halichondria, water passes more rapidly from ostia to oscula in the presence of ambient currents; water flows through freshly killed Halichondria only when the surrounding medium is in motion. 3. Quantitatively similar passive flow may be induced by motion of the medium in a variety of physical models of sponges. 4. The efficacy of the induction of passive flow depends in a complex manner on oscular geometry although, in general, sharp-edged, protruding apertures are the most effective exits. 5. These apparently novel results are consistent with much earlier literature on the structure, behavior, and ecology of sponges.