The earliest crustacean-annelids possessed large labra or prostomia projecting backward, still retained in the Apodidæ and trilobites. This labrum almost necessitated a very deliberate manner of browsing. The animal would creep along, and would have to run some way over its food before it could get it into its mouth, the whole process, it seems to us, necessitating a number of small movements backwards and forwards. Small living prey would very often escape, owing to the fact that the animal's mouth and jaws were not ready in position for them when first perceived. The labrum necessitates the animal passing forwards over its prey, then darting backward to follow it with its jaws. We here see how useful the gnathobases of _Apus_ must be in catching and holding prey which had been thus passed over. Indeed the whole arrangement of the limbs of _Apus_ with the sensory endites forms an excellent trap to catch prey over which the labrum has passed. In alcoholic specimens of _Apus_ the labrum is not in a horizontal plane, as it is in most well preserved trilobites, but is tipped down at an angle of from 30 to 45, and the big mandibles lie under it. It has considerable freedom of motion and is held in place by muscles which run forward and join the under side of the head near its posterior margin. It seems entirely possible that the hypostoma of the trilobite had as much mobility as the labrum of _Apus_, and that byopening downward it brought the mouth lower and nearer the food. It will be recalled that the hypostomata of practically all trilobites are pointed at the posterior margin, there being either a central point or a pair of prongs. By dropping down the hypostoma until the point or prongs rested on or in the substratum, and sending food forward to the mouth by means of the appendages, a trilobite could make of itself a most excellent trap, and if the animal could dart backward as well as forward, the hypostoma would be still more useful. There is no reason to suppose that they could not move backward, and the "pygidial antennæ" of _Neolenus_ indicate that animals of that genus at least did so. This habit of dropping down the hypostoma would also permit the use of those anterior gnathobases which seem too far ahead of the mouth in the trilobites with a long hypostoma.