The lophophore, a ring of ciliated tentacles, protrudes from the polypide to feed. When not feeding, the lophophore retracts into the polypide through the tentacular sheath. The cilia of the lophophore can change direction to eject isolated particles of indigestible matter. The classification in suborders is based upon frontal calcification and the mechanism of lophophore protrusion. The mouth is at the base of the lophophore. The gut is U-shaped with the anterior mouth at the center of the lophophore. The lophophore retractor is the muscle which controls the movement of the lophophore. In the Bryozoa it is outside the ring of the lophophore. The lophophore surrounds the mouth and is an upstream collecting system for suspension feeding. The colour varies but is usually orangish-brown with a red, orange or green lophophore. The lophophore support is loop shaped in contrast to the spiralia of similar looking spiriferids. The lophophore and mantle are the only surfaces that absorb oxygen and eliminate carbon dioxide.