Geneka of the N'orth American Palaeozoic Bryozoa. 443 



the ciliary whirlpool can well be imagined. Unless we can sup- 

 pose that a peculiar diet is necessary for the species furnished 

 with the preheDsile appendage, it is hardly probable that the 

 ordinary arrangements would have to be supplemented by the 

 service of such uncertain purveyors. And should they be feed- 

 ers on dead organisms only (as has been suggested), they would 

 certainly lead a precarious existence if dependent on the chance 

 supplies of the avicularian commissariat. The appendages, it 

 must be remembered, have no freedom of movement; they do 

 not go in quest of prey ; they merely oscillate, without variation, 

 to and fro, snapping their jaws at haphazard, or when aroused 

 by some irritation of the tactile setae. Their captures must be 

 fitful and uncertain, and if the food requires long keeping to be 

 fit for use (and under the conditions this seems to be a necessary 

 supposition), the colony must be in a chronic condition of famine. 

 If living animals be the required diet, then the cilia are adequate 

 to the supply of them, and the avicularia are not. 



'' On the whole (though the question is involved in much ob- 

 scurity), I am inclined to regard the avicularia as charged with 

 a defensive rather than an alimentary function. They may 

 either arrest or scare away unwelcome visitors. Their vigorous 

 movements and the snapping of their formidable jaws may have 

 a wholesome deterrent effect on loafing annelids and other va- 

 grants, whilst the occasional capture of one of them may help 

 still further to protect the colony from dangerous intrusion. On 

 this view of them, they have a function analogous to tnat of the 

 other appendage with which the Cheilostomata are furnished. 

 The vibraculum, though morphologically related to the zooecium 

 like the avicularium, is more immediately connected with the 

 latter ; and we find a line of transition forms linking the two to- 

 gether. It consists, in its more perfect condition, of a chamber, 

 in which the muscles are lodged, and a movable bristle, sus- 

 pended in a kind of cleft at its upper extremity, in which it 

 works backward and forward. The seta (or bristle) is broad at 

 the base and above it slender, and often of considerable length. 

 In some cases it attains an enormous development, and forms 

 either a whip-like appendage or an organ of such a size and 

 strength as to be available for locomotive purposes. On the 

 lower part of the wall of the chamber there is always a small 



