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 pedicellarias 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- 

 * " Histoiy of the British Zoophytes," 2nd edit. voL i. p. 259. 



