200 THE MECHANICAL FUNCTIONS. 



sometimes buried to a considerable depth, and at others have 

 several inches of their tubes exposed. In the one case, they 

 must work their way speedily to the surface; in the other, 

 they must dive deeper below it. The manoeuvres of the 

 terebella are best observed by taking it out of its tube and 

 placing it under water upon sand. It is then seen to unfold 

 all the coils of its body, to extend its tentacula in every di- 

 rection, often to a length exceeding an inch and a half, and 

 to catch, by their means, small fragments of shells, and the 

 larger particles of sand. These it drags towards its head, 

 carrying them behind the scales which project from the an- 

 terior and lower part of the head, where they are immediate- 

 ly cemented by the glutinous matter which exudes from that 

 part of the surface. Bending the head alternately from side 

 to side, while it continues to apply the materials 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 in- 

 teresting. For the purpose of fixing the different fragments 

 compactly, it presses them into their places with the erected 

 scales, at the same time retracting the 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 poste- 

 rior 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 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 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 considerabl}^ slower. 



