INTRODUCTION. 



necessary to avoid the descriptive terms which 

 naturally be suggested by the organization and apparent 

 function of the true "bird's head." The latter would 

 seem to be a grasping organ; but in a large proportion 

 of the lower forms there is nothing that can properly be 

 called a " beak/' whilst the equivalent of the mandible 

 is utterly inefficient for prehensile purposes. 



Every avicularium consists of a chamber, of variable 

 size and shape, in which is lodged an apparatus of muscles, 

 of a movable horny appendage which is worked back- 

 wards and forwards by the muscles, and of a fixed frame 

 opposed to it surrounding an aperture, upon which it falls 

 when closed. In many cases, if not in all, the chamber 

 also contains a cellular body, which is in all probability 

 the homologue of a polypide. 



These elements may compose a structure very closely 

 resembling the ordinary zooecium ; or they may be so 

 modified as to constitute an articulated and prehensile 

 appendage, armed with curved beak and powerful jaws, 

 and provided with a delicate tactile organ such as we find 

 in the genus Bugula. In all cases the avicularium is to be 

 regarded morphologically as a metamorphosed zooecium*, 

 though in its more complex forms there is little to betray 

 its lineage. 



Amongst our British Polyzoa we find this zooidal form 



* In the zooecium of the Cheilostomata the following parts may be dis- 

 tinguished, which should be borne in mind in studying the history of the 

 avicularium : (i.) the chamber, in which the polypide is lodged (Wood- 

 cut, fig. xxx. c) ; (ii.) a movable corneous plate (the operculuni) which closes 

 the entrance to it (fig. xxx. o) ; (iii.) two sets of muscles, by which this valve 

 is opened and shut ; (iv.) the aperture, an opening covered by membrane, 

 which in many species occupies a considerable portion of the front surface 

 of the cell (fig. xxx. a). In other cases this is wanting, and is replaced by 

 a solid calcareous wall. The mandible of the aviculnrium, under all its 

 modification*, is the homologue of the opercslum. 



