AVICULARIA AND VIBRACULA. 109 



In both Bowerbanlcia and Liclienopora the zooids of each colony are 

 very similar one to the other ; in other genera, however, especially in 

 Bugula, a distinct polymorphism seems to' take place. Bugida lives in 

 the Laminarian and Coralline Zones and except at very low spring 

 tides can only be taken in the dredge. It is fairly abundant in British 

 waters, and grows in loose spiral coils of great elegance. Examining 

 a spray, we find the zooecia or cells are arranged in tabular manner, 

 edge to edge, the apertures all on the inner surface. The normal 

 zooids approach closely to the form of those of Bowerbanlcia, but what 

 arrests the attention at once is the presence of peculiar beak-like bodies 

 scattered over the surface. Each is a small rounded object provided 

 with a curved upper beak, to which is hinged a movable jaw-like 

 mandible, constantly snapping viciously. The whole so closely re- 

 sembles a bird's head and beak that it has received the name of 

 avicularium. The only internal organs are two powerful muscles used 

 to set the mandible in motion. According to many authorities, these 

 avicularia are believed to be zooids modified for a highly specialised 

 function, but this point is not assured. What the exact function is, is 

 also doubtful. The more likely theory is that they are protective, 

 scaring minute predatory creatures away by the constant gnashing of 

 the jaws ; some, however, have argued that they assist in procuring 

 food. Thus they may sometimes be seen holding some minute worm 

 or crustacean in their grip and while such object is useless, from its 

 size, as food, yet if held till decay begins, the disintegrated particles 

 may be swept into the mouth by the waving of the tentacular cilia. 

 Among my microscopical preparations I have now one wherein an 

 avicularium is holding the limb of a small Amphipod. 



Whip-like organs, called Yibracula, are also seen in Scrupocellaria 

 and other Bryozoa, and again are thought by many to be modified 

 individuals bereft of alimentary, reproductive and other organs except- 

 ing muscles. The vibracula most probably also subserve a protective 

 function, their constant lashing keeping unwelcome visitors at a 

 respectful distance and also freeing the surface of the colony from 

 decaying or otherwise undesirable matter. It may also be that the 

 lashing serves to keep the water around in healthful motion and so to 

 bring new food particles within reach of the ciliary vortices. 



Divisions of the Gymnol^emata : — 



Sub-Order a. — Gyclostomata. Zooecial orifices round, without avicu- 

 laria or vibracula or opercular apparatus, e.g., Crista, Lichenopora. 



Sub-Order b. — Ctenostomata. Zooecial orifices closed upon retraction 

 by folds of the tentacular sheath or by a circlet of spines, e.g., 



