The most important function of the bird's head avicularium of Bugula simplex and B. stolonifera is that of reducing the tube-building activities of amphipods. These avicularia would probably also be effective in reducing predation by animals of such a size and shape that could be readily seized. That the avicularium is well adapted to such functions is indicated by: 1) the structurally sound design that maximizes the force that can be applied for the least amount of material; 2) the arrangement of the musculature that increases the ability to grasp objects of small diameter; 3) the reaction to mechanical stimuli, namely, nodding and closing the mandible; 4) the placement of avicularia on the colony in positions where they can easily seize crawling organisms. Animals effectively seized by avicularia are those that crawl over the colony and are 0.5 to 4 mm long with either many appendages or wormlike bodies less than 0.05 mm in diameter. Most animals outside this range of size and shape, including most potential predators and larvae of fouling organisms, are inhibited very little by Bugula-type avicularia.