SEA-MA T MAKERS. 305 



moveable bird's-head process attached to each of the 

 cells of the sea- mat (f lustra avicularis}"^ 



Mr. Hincks has traced the modification of the 

 avicularium and vibraculum, as these processes are 

 called, in a large number of species, and came to the 

 conclusion that they are, morphologically, to be 

 regarded as metamorphosed polype-cells (Zocecia). 

 " Every avicularium," he contends, " consists of a 

 chamber, of variable size and shape, in which is 

 lodged an apparatus of muscles, of a moveable horny 

 appendage, which is worked backwards and forwards 

 by the muscles, and of a fixed frame opposed to it, 

 surrounding an aperture, upon which it falls when 

 closed. In many cases, if not all, the chamber also 

 contains a cellular body, which is in all probability 

 the homologue of a polypide." It is hardly necessary 

 to follow him through the various modifications of 

 these appendages, although casually there would 

 appear to be a very great difference, almost devoid 

 of any similarity of appearance, between the lowest 

 and most imperfectly developed forms, and the 

 genuine bird's-head processes. 



" The function of the avicularia," he says, " it is diffi- 

 cult to determine, nor, indeed, can the same function 

 be assigned to all of them. The primary forms are 

 many of them quite unfit for prehensile work. The 



Johnston, "British Zoophytes," vol. i. p. 333. 

 X 



