INTRODUCTION. 



mandible is prolonged and attenuated, whilst the beak is 

 almost rudimentary. Indeed it is difficult to draw the 

 line between them, unless we regard the total absence 

 of a distinct beak as the essential characteristic of the 

 vibraculum. We have a case of the slight extension and 

 attenuation of the mandible in Schizoporella spinifera 

 (Plate XXXV. fig. 7) ; in Membranipora ciliata the change 

 is occasionally carried still further; but there is great 

 variability, and the mandible is now of the ordinary form, 

 and now prolonged into a vibracular process (Plate 

 XXVIII. fig. 8). In Schizoporella vulgaris the man- 

 dible is metamorphosed into a seta, but the beak survives, 

 and the movement is probably nothing more than the 

 rising and falling, as of a lid. In Mastigophora Hynd- 

 i n n a n i (Plate XXXVII. fig. 5) the mouth is so modified 

 as to give much more play to the seta, which is thrown 

 backwards and forwards with perfect freedom, and has 

 much the appearance of a lash. In this species the 

 vibraculum is borne on a distinct cell, resembling the 

 zooecium (on which it is developed) except in size. In 

 yet another case the vibracular cells are still further 

 reduced, and one is placed on each side of the orifice of 

 the zooecium (Plate XXXVII. fig. 1). When we come 

 to the higher forms we meet with cases in which the seta 

 resumes the dimensions of the mandible, and loses its 

 free and vigorous swing (Plate VI. fig. 8). 



The most elaborate form of this appendage is found in the 

 genus Caberea. There the chamber is large, and traversed 

 on the upperside by a channel or groove, in which the seta 

 lies when at rest*. The latter is of great length, and ser- 



* This groove must be regarded as a modification of the aperture, t. e. of 

 that portion of the front wall of the cell which, in Caberea and many other 

 forms, is filled in by membrane. 



