"^^'^ Eefoet op the State Geologist. 



vigorous swing on its jointed base, grotesque both in form and 

 movement. 



" But in a large proportion of cases the appendage exhibits 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 articu- 

 lated ^bird's head' bears no resemblance to its associated struc- 

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

 alien form and has parted with all the familiar features of its 

 tribe; its aspect and habits are those of a foreigner, and as we 

 watch it even with its genealogy in our hands, we cease to won- 

 der that it remained so long a mystery and a puzzle to the zoolo- 

 gist. When we come to consider the avicularium, not merely in 

 its more complex 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 zo- 

 cecium, 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 form 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 essential ele- 

 ments of its structure; and in doing so, it will be necessary to 

 avoid the descriptive terms which might naturally be suggested 

 by the organization and apparent function of the true 'bird's 

 head.' The latter would be a grasping organ, but in a large 

 proportion of the lower forms there is nothing which can prop- 

 erly be called a ' beak,' whilst the equivalent for 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 moved backward and forward by the action of the ' 

 muscles, and of a fixed frame opposed to it, surrounding an aper- 

 ture 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. 



