BIRD'S-HEAD ORGAN. 227 



would be the upper, though here reversed) is truncate, 

 except the sharp-curved point, and encloses a capa- 

 cious cavity ; within this, the lower mandible, which 

 is curved and pointed in like manner, is jointed, and, 

 working on its hinge with an enormous range, shuts 

 with a snap into the upper. 



There are species (such as Bicellaria ciliata, Aca- 

 marchis flabellata, and others) in which the resent 

 blance to the entire skull of a bird is most marked 

 and striking. In these two the organ is not sessile 

 and fixed, but attached by a hinge-joint, which per- 

 mits great freedom of motion. A shelly knob is 

 placed on the outside of the cell, and on this is seated 

 the mimic skull, at that point where naturally it would 

 rest on the atlas-joint of the vertebrae. The union is, 

 as I have said, by a freely-working hinge-joint ; and 

 thus the whole skull sways backward and forward, 

 just as a head does upon the spine. 



But besides this, the form of the two mandibles is 

 (in the former of the last-named two species, for in- 

 stance) a far more perfect copy of a vulture's beak. 

 The vraisemblance is, indeed, most wonderful ; and 

 the microscope can scarcely present a more striking 

 spectacle than one of these shrubby Polyzoa in full 

 health and vigour in a trough of sea-water. The eye 

 is bewildered and the mind amazed at the sight of 

 scores of naked skulls swinging to and fro, not evenly 

 and uniformly, but fitfully, and, as it appears, wil- 

 fully ; while the yawning gape of the mandibles to an 

 awful reach, and ever and again the spiteful snapping 



