320 THE INHABITANTS OF THE SEA. 
polyzoon, these tiny “ vulture-heads,” which are either sessile 
or pedunculated, keep up a continual motion, and it is most 
amusing to see them see-sawing and snapping and opening 
their jaws, and then sometimes in their incessant activity even 
closing upon the beaks of their neighbours. 
It is still very doubtful what is their precise function in the 
economy of the animal; whether it is to retain within reach of 
the ciliary current bodies that may serve as food, or whether it 
is like the pedicellariz of the sea-urchins to remove extraneous 
particles that may be in contact with the surface of the poly- 
zoary. The latter would seem to be the function of the  vibra- 
cula,” which are likewise pretty generally distributed among the 
polyzoa. Each of these long bristle-shaped organs, springing 
at its base out of a sort of cup, that contains muscles by which 
it is kept in almost constant motion, sweeps slowly and carefully 
over the surface of the polyzoary, and removes what might be 
injurious to the delicate inhabitants of the cells, when their 
tentacles are protruded. So carefully have these lowly molluscs 
been provided for! 
The polyzoa can neither hear nor see, at least as far as we are 
able to ascertain, but the delicacy of their sense of touch is very 
great. ‘ When left undisturbed in a glass of fresh sea-water,” 
says Dr. Johnston,* “they push their tentacula beyond the 
mouth of the cell by straightening the body, and then expand- 
ing them in the form of a funnel or bell, they will often remain 
quiet and apparently immovable for a long time, presenting a 
very pretty and most interesting object to an observer of the 
‘minims of nature.’ If, however, the water is agitated, they 
withdraw on the instant, probably by aid of the posterior liga- 
ment or muscle ; the hinder part of the body is pushed aside up 
the cell, the whole is sunk deeper, and by this means the tenta- 
cula, gathered into a close column, are brought within the cell, 
the aperture of which is shut by the same series of actions. 
The polyzoa of the same polyzoary often protrude their thou- 
sand heads at the same time, or in quick but irregular succes- 
sion, and retire simultaneously, or nearly so, but at other times 
I have often witnessed a few only to venture on the display of 
their glories, the rest remaining concealed, and if, when many 
are expanded, one is singled out and touched with a sharp in- 
* “History of the British Zoophytes,” 2nd edit. vol. i. p. 259. 
