The fundamental constraint shaping animal systems for internal gas transport is the slow pace of diffusion . In response, most macroscopic animals have evolved systems for driving internal flows using muscular pumps or cilia. In arthropods, aside from terrestrial lineages that exchange gases via tracheal systems, most taxa have a dorsal heart that drives O2-carrying hemolymph through peripheral vessels and an open hemocoel , with O2 often bound to respiratory proteins. Here we show that pycnogonids (sea spiders), a basal group of marine arthropods , use a previously undescribed mechanism of internal O2 transport: flows of gut fluids and hemolymph driven by peristaltic contractions of a space-filling system of gut diverticula. This observation fundamentally expands the known range of gas-transport systems in extant arthropods.