The internal, living portion of the zooid is known as the polypide, whose walls are formed by the outer epidermis and inner peritoneum. 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 lophophore is controlled by the zooid's nervous system, which consists of a ganglion at the lophophore base. This ganglion is responsible for motor and sensory impulses to and from the lophophore, as well as the epithelium and digestive tract. The lophophore retractor is the muscle which controls the movement of the lophophore.