PROCESS OF TUBE-BUILDING. 303 



to a length exceeding an inch and a half; and they are 

 soon seen dragging in small fragments of shells and 

 the larger particles of sand. These it places behind 

 the scales which project from the anterior and lower 

 part of its head, where they are immediately cemented 

 together by a gluten which exudes from the animal. 

 Bending the head from side to side while it continues 

 to apply the materials of its tube,, the Terebella very 

 soon has formed a complete collar, which it employs 

 itself in lengthening regularly from every part of its 

 circumference, with a persevering activity highly in- 

 teresting to behold. In order to fix the fragments 

 compactly, it presses them into their places with the 

 erected scales, at the same time slightly retracting 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. 



When the tube is from half an inch to an inch in 

 length, the animal proceeds to burrow. Directing its 

 head against the sand, it slowly penetrates it. The 

 penetrating force is very inconsiderable, because the 

 creature, having to form its case as it descends, can 

 advance but slowly. It is now of course obliged to 

 avail itself of the materials with which it may come 

 in contact. Its progress is marked by the gradual 

 disappearance of the tail, which continues to with- 

 draw into the tube till the whole has entered. Soon 

 afterwards the conchilega is found to have turned in 

 its abode, and its head appears at the surface. 



In summer the whole task is completed in four or 

 five hours; but in cold weather, when the animals are 



