104 MISCELLANEOUS OBJECTIONS TO THE [Chap. VII. 



across the object-glass of my microscope. When a branch was 

 placed on its face, the vihracula became entangled, and they made 

 violent efforts to free themselves. They are supposed to serve as 

 a defence, and may be seen, as Mr. Busk remarks, "to sweep 

 slowly and carefully over the surface of the polyzoary, removing 

 what might be noxious to the delicate inhabitants of the cells when 

 their tentacula are protruded." The avicularia, like the vibracula, 

 probably serve for defence, but they also ca'xh and kill small living 

 animals, which it is believed are afterwards swept by the currents 

 within reach of the tentacula of the zooids. Some species are 

 provided with avicularia and vibracula ; some With avicularia alone, 

 and a few with vibracula alone. 



It is not easy to imagine two objects more widely different in 

 appearance than a bristle or vibraculum, and an avicularium like 

 the head of a bird ; yet they are almost certainly homologous and 

 have been developed from the same common source, namely a zooid 

 with its cell. Hence we can understand how it is that these 

 organs graduate in some cases, as I am informed by Mr. Busk, 

 into each other. Thus with the avicularia of several species of 

 Lepralia, the moveable mandible is so much produced and is so like 

 a bristle, that the presence of the upper or fixed beak alone serves 

 to determine its avicularian nature. The vibracula may have been 

 directly developed from the lips of the cells, without having passed 

 through the avicularian stage ; but it seems more probable thai 

 they have passed through this stage, as during the early stages cf 

 the transformation, the other parts of the cell with the included 

 zooid could hardly have disappeared at once. In many cases the 

 vibracula have a grooved support at the base, which seems to repre- 

 sent the fixed beak ; though this support in some species is quite 

 absent. This view of the development of the vibracula, if trust- 

 worthy, is interesting ; for supposing that all the species provided 

 with avicularia had become extinct, no one with the most vivid 

 imagination would ever have thought that the vibracula had originally 

 existed as part of an organ, resembling a bird's head or an irregular 

 box or hood. It is interesting to see two such widely different 

 organs developed from a common origin ; and as the moveable lip 

 of the cell serves as a protection to the zooid, there is no difficulty 

 in believing that all the gradations, by which the lip became con- 

 verted first into the lower mandible of an avicularium and then 

 into an elongated bristle, likewise served as a protection in different 

 ways and under different circumstances. 



In the vegetable kingdom Mr, Mivart only alludes to two cases, 



