188 TEREBELA. 



by doubling the body. After the tentacula turn the head follows, and 

 the body folding on itself, the anterior extremity is liberated where the 

 tentacula begin to protrude. 



The worker can invest part of the body, even the whole, with sand 

 by adhesion, but not as a tube, for this edifice requires a peculiar kind 

 of construction. 



A subsisting tube is prolonged one or two inches in the course of a 

 night, though not with equal solidity under all circumstances, whether 

 from scanty materials, defective secretions, or the disposition of the 

 architect. In the natural state, the tube seems to be always directed 

 horizontally. No parts, perhaps, unless the orifices, are vertical. In con- 

 finement the Terebella inclines to build upwards, at least such a ten- 

 dency is then more evident. But it may result from the limited sphere 

 of operation. 



The fashion of the architect is subject to capricious modifications, 

 sometimes it formed a vertical cylinder unsupported, though composed of 

 very slight materials ; also it appeared from a long and tortuous tube 

 ascending the side of a glass jar, that, 1. It was cylindrical for a consi- 

 derable distance from the foundation ; that, 2. The animal then began to 

 economise its labour, by appropriating the glass as one side of the rising 

 edifice ; that 3. On being conducted in this form to the highest edge of 

 the vessel, which was immerged in one more capacious, the cylindrical 

 form was resumed, because the substituted support by the side of the 

 glass was lost ; that, 4. When the continued extension of the tube at 

 length reached the side of the larger vessel, the animal, to spare its 

 labour, again availed itself of the side of the glass, and now directed the 

 formation of the edifice downwards. 



Though the creature commonly prosecutes its advantage as far as 

 possible for saving labour, when the structure reaches a solid wall, this 

 advantage is often abandoned as useless where a perfect cylinder might 

 be constructed in actual contact with that wall, or within a hair's breadth 

 of it. Likewise, after the cylinder has been advanced indefinitely, a 

 new orifice is broken through the side, and a branch from it prolonged in 

 different directions from that of the original edifice, where, in process of 



