Genera of the !N'okth American Palaeozoic Bryozoa. 437 



" These elements may compose a structure very closely resem- 

 bling an ordinary zooecium ; or they may be so modified as to 

 constitute an articulate 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 metamor- 

 phosed zooecium, though in its more complex form there is little 

 to betray its lineage. 



^'AmoDffst our British Polyzoa Ave find this zooidal form in its 

 most rudimentary condition in such genera as Flustra and Cel- 

 laria. Here it is not a specialized structure attached to one of 

 the zoa3cia ; it occupies the place of one of them in the colony. 

 It consists of a dwarfed cell, on the upper surface of which is 

 placed the usual oral valve, but which is destitute of a polypide ; 

 at the same time the valve is frequently of unusual and dispro- 

 portionate size, and occupies a large part of the area of the cell. 

 Except in size, however, it has undergone but little change, al- 

 though a certain variation of form already indicates its plas- 

 ticity. In one species {Cellaria sinuosa) it assumes a triangular 

 shape ; in the common C. fistulosa it is almost indistinguishable 

 from the ordinary operculum. The degree in which the avicu- 

 larian chamber (or cell) is reduced in size varies greatly amongst 

 these primitive and rudimentary forms. In Cellaria Johnsoni it 

 is a miniature copy of the normal zooecium, almost its only pe- 

 culiarity being the elevation and somewhat increased size of the 

 operculum. In other cases the atrophy of the cell is carried to a 

 great extent and the operculum occupies almost the whole of the 

 area. 



"As specialization proceeds, the chamber is minimized and the 

 adaptive modification of the valve becomes more and more varied 

 and elaborate. In the mandible of the ' bird's head ' appendages 

 it reaches its climax, whilst in this form the zooecium itself has 

 lost every trace of its original character and function, and merely 

 lodges the machinery by which the curious prehensile instrument 

 is worked. 



" Nowhere, perhaps, is the relation of the avicularium to the 

 normal zooecium more clearly traceable than in a foreign species 

 of Membranipora (as yet I believe undescribed), in which a very 

 striking modification of the operculum is combined with the 



