a small constellation north of aquila; the arrow.-the keystone of an arch.-the distance from a point in a curve to the chord; also, the versed sine of an arc; -- so called from its resemblance to an arrow resting on the bow and string.-the larger of the two otoliths, or ear bones, found in most fishes.-a genus of transparent, free-swimming marine worms having lateral and caudal fins, and capable of swimming rapidly. it is the type of the class chaetognatha.