196 TEREBELLA. 
kneading masses in its mouth, disgorging them successively, or in polish- 
ing the rude workmanship resulting from its labours. 
A perfect tube is half an inch in external diameter, the sides being 
at least an eighth of an inch thick. It is wider within than the size of 
body, to admit of reversal, like the former. From the dimensions it 
appears massive and clumsy, yet in fact so fragile as to be incapable of 
sustaining its own weight. From the fragility of their tubes, the Tere- 
bella: in general seek adhesion to solid substances for support, often con- 
ducting them horizontally. Likewise, both extremities of the artificial 
dwellings of the Terebella are open, a characteristic whereby they are 
distinguished from those of the genus Amphitrite. The same peculiari- 
ties of fabrication, in a cylindrical form, of rising vertically, or assuming 
a horizontal direction where the space is free, or of restriction to an im- 
perfect cylinder when ascending a vessel's side, are demonstrated by this 
as by the preceding species. 
The circuit bemg free, a complete uninterrupted revolution is per- 
formed by the architect while constructing its tube. *The cylindrical 
form results from the revolution, not from the shape of the body, among 
those animals framing artificial dwellings. 
An imperfect cylinder is constructed much more speedily on the 
side of a vessel, than a perfect cylinder in the centre. The artist does 
not perform a full revolution, though capable of doing so, while fabri- 
cating the segment of a cylinder. That would be superfluous. Having 
discharged its compound to the left, it continues towards the right, in- 
stead of passing around by the vacant side of the glass. Thus it pursues 
a retrograde movement, instead of describing a complete circle. Its 
operations are conducted from left to right, and from right to left alter- 
nately, without,covering the internal surface of the glass, whereby there 
is a saving of labour, as well as of material. 
Shall this be accounted the demonstration of a higher degree of in- 
stinct, warning the creature against superfluous work ? Or does it rseult 
simply from the configuration of its body ? The latter is not evident, for 
the cylindrical form may be resumed apparently at will. But the influ- 
