250 THE MECHANICAL FUNCTIONS. 



the surface. Bending the head alternately from 

 side to side, while it continues to apply the mate- 

 rials of its tube, the terebella has very soon formed 

 a complete collar, which it sedulously employs 

 itself to lengthen at every part of the circumference 

 with an activity and perseverance highly inter- 

 esting. For the purpose of fixing the different 

 fragments compactly, it presses them into their 

 places with the erected scales, at the same time re- 

 tracting its body. Hence the fragments, being 

 raised by the scales, are generally fixed by their 

 posterior edges, and thus overlaying each other, 

 often give the tube an imbricated appearance. 



Having formed a tube of half an inch, or an 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 slight 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 accom- 

 plished, 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 summer 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. 1*29), proceeding 

 from the margin of the mouth and other parts of 



