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, im process of 
