280 THE MECHANICAL FUNCTIONS. 



inch in length, the terebella proceeds to burrow; 

 for which purpose it directs its head against the 

 sand, and contracting some of the posterior rings, 

 effects a sUght extension of the head, which thus 

 slowly makes its way through the mass before it, 

 availing itself of the materials which it meets 

 with in its course, and so continues to advance 

 till the whole tube is completed. After this has 

 been accomplished, the animal turns itself within 

 the tube, so that its head is next to the surface, 

 ready to receive the water which brings it food, 

 and is instrumental in its respiration. In sum- 

 mer, the whole task is completed in four or five 

 hours ; but in cold weather, when the worm is 

 more sluggish, and the gluten is secreted more 

 scantily, its^ progress is considerably slower. 



Tentacula of various kinds are also met with 

 in several of the more active and vivacious kinds 

 of annelida, such as the Nereis (Fig. 129), pro- 

 ceeding from the margin of the mouth and other 

 parts of the head. This animal swims with 

 great facility by rapid, undulating inflexions of 

 its body ; and by practising a similar succession 

 of movements in the loose sand at the bottom 

 of the water, it quickly buries itself, and even 

 travels to considerable distances through the 

 sand, first extending the anterior rings, and then 

 bringing up the posterior part of the body ; its 

 progress being also much assisted by the action 

 of its numerous bristly feet.* 



* Osier, Phil. Trans. 1826. p. 342. 



