INTRODUCTION. 



tively rare occurrence. In its most highly specialized 

 forms, it is placed on the dorsal surface of the zooecium, 

 and the movable seta (which when at rest is laid back 

 upon the chamber) is swung round at intervals to the 

 front of the cell, sweeping slowly over the surface as if 

 to remove all noxious matter, and then returns to its ori- 

 ginal position. This movement goes on uninterruptedly 

 during the life-time of the colony ; and there can be no 

 doubt that its object is to clear away dangerous intruders 

 or accumulations of refuse from the neighbourhood of the 

 orifice. 



We have no difficulty in recognizing the equivalent of 

 the avicularian mandible and the operculum of the cell in 

 the seta. The mouth is here modified in the same sense 

 as the rest of the structure : the raised " beak " is absent, 

 being no longer useful ; but the margin is carried out 

 above into two prominent points, just within which the 

 bristle is articulated, clear of all hindrances, and so as to 

 possess the utmost freedom of movement (Woodcut, fig. 

 xli. pp). 



The homology of the parts becomes more evident when 

 we study the transitional forms. We meet with a develop- 

 mental stage (corresponding to the primary avicularium) , 

 in which the vibraculum is developed on the original 

 plane of the colony, and occupies the position of an ordi- 

 nary cell ; in some species the vibracular cells alternate 

 regularly with the zooecia. In such cases the movements 

 of the seta are of necessity much restricted, and the ap- 

 pendage is rather a servant of the colony than of the in- 

 dividual polypide (Plate XXI. fig. 7). 



The direct links between the vibraculum and avicula- 

 rium are found in those forms of the latter in which the 



9 



