OBJECTIONS TO THE THEORY OF NATURAL SELECTION 197 



Besides the avicularia, the polyzoa possess curious organs called 

 vibracula. These generally consist of long bristles, capable of 

 movement and easily excited. In one species examined by me the 

 vibracula were slightly curved and serrated along the outer mar- 

 gin, and all of them on the same polyzoary often moved simul- 

 taneously; so that, acting like long oars, they swept a branch 

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

 was placed on its face, the vibracula 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 catch and kill small liv- 

 ing animals, which, it is believed, are afterward swept by the cur- 

 rents within reach of the tentacula of the zobids. 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 movable 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 that 

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

 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 rep- 

 resent 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 origi- 

 nally 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 dif- 

 ferent organs developed from a common origin; and as the mov- 

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



