INTRODUCTION. 



Ixvii 



in its most rudimentary condition in such genera as Flustra 

 and Cellaria. Here it is not a specialized structure at- 

 tached to the zooecia ; 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 disproportionate size^ and oc- 

 cupies a large part of the area of the cell. Except in 

 size, however, it has undergone but little change, though 

 a certain variability of form already indicates its plasticity. 

 In one species {Cellaria sinuosa) it assumes a triangular 



Fig. xxix. 



a 



Vrimary Avicularia. 

 a. Cellaria sinuosa. b. C. fistidosa. e. Membranipora cornigera. 



d. M. arctica. 



shape ; in the common C. fistidosa it is almost undistin- 

 guishable from the ordinary operculum. The degree in 

 which the avicularian 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 (see Plate XIII. figs. 11, 12), almost 

 its only peculiarity being the elevation and somewhat in- 

 creased 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. 



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