OR FRESH WATER POLYZOA. 63 
can follow them; and it is a curious fact, that after the 
movement is completed, and they are safely ensconced, 
the fibres are not content to rest, but still keep up a 
lively motion, writhing and twisting like bundles of mi- 
nute worms. 
The tentacles all the while lie gathered closely togeth- 
er in the sheath, formed for them by the tube, which has 
been doubled upon itself inside of the cell, like the 
finger of a glove inverted within the empty palm. When 
once more ready to emerge, the opening of the cell, which 
has been contracted by a circular band of muscle, like the 
mouth of a bag drawn up with a string, relaxes and per- 
mits the ends of the tentacles to protrude. These warily 
search for the cause of the previous alarm, and, if no hos- 
tile movements betray the presence of an enemy, the 
whole bundle slowly and cautiously follows, halts a mo- 
ment, and then confidently unfolds its circlet of sentient 
threads. The Polyzoén reasons from the impression 
made upon these feelers, and cannot be induced to expose 
itself until thoroughly satisfied, by their exquisite sense 
of touch, that no danger lurks near its retreat. 
a degree of complication in their relations, both social and 
physical, which the simplicity of the organization, and the 
to the philosophical observer, _ 
The wonders revealed in the structure of these lovely 
dwellers in the perennial shadows of our fresh waters, 
tempt one to linger, but the history of their circulatory 
