INTRODUCTION. 1XV 



Bicellaria. This is the true "bird's head," an articulated 

 appendage attached to the zocecium, with a formidable 

 hooked beak and a mandible worked by powerful muscles, 

 perpetually snapping its jaws with monotonous energy, and 

 swaying to and fro with vigorous swing on its jointed 

 base grotesque both in form and movement. 



But in a large proportion of cases the appendage ex- 

 hibits a much simpler structure, and is totally destitute 

 of the peculiar shape which has suggested its name. It 

 is necessary to study its morphology in extenso to obtain 

 a clue to its history ; the articulated " bird's head " bears 

 no trace of resemblance to the associated structures, 

 which are yet undoubtedly of its kin ; it has assumed an 

 alien form, and has parted with all the familiar features 

 of its tribe ; its aspect and its habits are those of a 

 foreigner ; and as we watch it, even with its genealogy in 

 our hands, we cease to wonder that it remained so long a 

 mystery and a puzzle to the zoologist. When we come 

 to consider the avicularium, not merely in its more com- 

 plex and highly organized condition, but in its totality, as 

 it is represented in a long series of gradational forms, we 

 are left in no doubt as to its structural affinities. We 

 can trace the course of its development from the first 

 rudimentary stages, which are hardly distinguishable from 

 the ordinary zooecium, through a multitude of phases, up 

 to the highly elaborated prehensile appendage in which 

 no family likeness survives. And probably the best way 

 of presenting its history will be to begin with the lowest 

 forms in which it occurs, and to follow it through its chief 

 modifications up to the highest. 



It will be desirable, however, first to indicate the essen- 

 tial elements of its structure ; and in doing so, it will be 



