SECT. v. TUB1COLA. 155 



The mucus, which cements together the particles of 

 sand and shell for the artificial tubes of this kind of 

 worms, is believed to be secreted from glands in the 

 first segment of the body ; but the long slender filaments 

 of the head are the active agents in the structure. The 

 tentacles are hollow bands with strong muscular edges, 

 which the worm can bring together so as to form a 

 cylinder, at any point of which it can take up a particle 

 of sand, or a whole row of particles, and apply them to 

 its glutinous body. The fibres at the free ends of the 

 tentacles act both as muscular and suctorial organs ; for 

 when the worm is going to seize a particle of sand or food, 

 the extremity of the tentacle is drawn in by the reflux of 

 the colourless liquid in its interior, so that a cup-shaped 

 cavity is formed in which the particle is secured by 

 atmospheric pressure, aided by the power of the circular 

 muscular fibres at the extremity of the tentacle. 



The Serpula and its allies are richly-coloured worms, 

 living in contorted tubes with lids, frequently seen 



Fig. 134. Pushing poles of Serpula. 



encrusting rocks, the shells of oysters, and other inol- 

 lusca. By a peculiar mechanism of their bristly feet 

 they can open the lid of their tube, push out their fan 

 of gorgeous tentacles, pull it in again, and shut up the 

 tube. As the protrusion of the worm from its tube is 

 slow, cautious, and gradual, the retreat swift and sudden 

 as lightning, there are two distinct sets of organs in 

 the feet by which these motions are performed. 6 



6 Dr. Thomas Williams on ' British Annelides,' British Association, 1852. 



