As it goes on mostly at night, feeding is not often observed. The favored food seems to be annelids, and these have been seen to be swallowed whole, making a prominent bulge in the thin, elastic body of the nemertean. Mollusks, crustaceans, and fishes are also eaten, though bigger prey may be sucked at, not downed in one piece. Undigested residues do not have to be cast out the mouth, for the nemerteans are the lowest animals that have an anus, a second open- ing to the digestive tract, which voids materials from the rear end of the animal. The ribbon worms are built much like flatworms, but aside from the anus they can boast another important improvement. They have contractile blood vessels. Waves of contraction in the strong muscles of the body wall also help to push blood and food along their respective tubes, and in a worm at rest it is these powerful muscular waves that are seen to pass along the body.