Studies were carried out to identify the route by which macromolecules and large volumes of fluid traverse the skin of terrestrial gastropods. Electron micrographs of the skin of the banana slug Ariolimax columbianus demonstrated that carbon particles can enter large, specialized cells and pass thence to the exterior. These cells, which are termed channel cells, range up to 500 micron in length; they reach from the external surface of the skin to deep within the subepithelial interstitium. At the light-microscope level they show a large central channel or reservoir apparently filled with homogeneous fluid; after injection of ink into the body cavity this central channel becomes ink-filled. Electron micrographs show cisternae of the smooth endoplasmic reticulum, opening from the cell surface and occasionally traversing the entire cytoplasmic layer. The neurohormone arginine vasotocin stimulates fluid and particle movement through the channel cell; this response is inhibited by norepinephrine. Fluid output is dependent on the presence of a transwall hydrostatic pressure gradient of about 7 torr or above, as well as on activation of the channel cells.