sentences:  Fluid can be pumped through the gills' interiors by muscles associated with the lantern, but this does not provide a continuous flow, and occurs only when the animal is low in oxygen.Tube feet can also act as respiratory organs, and are the primary sites of gas exchange in heart urchins and sand dollars, both of which lack gills.The inside of each tube foot is divided by a septum which reduces diffusion between the incoming and outgoing streams of fluid.