CELLULARIID^. 31 



The avicularia are of the sessile and fixed type, and 

 exhibit, on the whole, a very constant and definite arrange- 

 ment. They are either disposed along the outer edge of 

 the cells or on the front surface. The lateral avicula- 

 ria are seldom absent, though where the median appen- 

 dages are highly developed, they are more or less atro- 

 phied, and in some cases dwindle into complete insignifi- 

 cance. They are either attached to every cell, or each 

 internode (in the jointed forms) carries a certain 

 number. 



The vibracula are less universally present ; but in cer- 

 tain sections of the family they attain a remarkable degree 

 of development, and exhibit, perhaps with one exception, 

 their most highly specialized form. 



The radical fibres, by means of which the tufted zoaria 

 are attached, exhibit many interesting modifications, cor- 

 responding strictly in some cases with the nature of the 

 habitat, and assuming at times in the same species, under 

 different circumstances, very difiPerent adaptive forms. I 

 have referred to some of these curious modifications in the 

 account of Scrupocellaria reptans'^. 



Though the family of the Cellulariidee is properly de- 

 scribed as rich in the secondary appendages, it possesses 

 in the genus Cellularia a representative which is chiefly 

 distinguished by the simplicity of its structure, and con- 

 nects it with the still simpler group of the Eucratiidce. 



In defining the limits of the present family, I have not 

 ventured to exclude the remarkable genus Caberea, 

 although it presents some very striking peculiarities, and 

 is distinguished from the rest of the tribe by the non- 

 articulated character of its zoarium. In other points of 



* These interesting adaptations are not confined to this family. A re- 

 markable instance of similar plasticity in the radical appendages is furnished 

 by Bugida plumosa. 



