220 
LECTURE XII. 
as to be sometimes perceptible by a sharp eye, 
even without a glass. It is remarkable for its 
strange power of reviviscence, or restoration to 
life and motion after being dried many months on 
a glass. The Wheel-i\nimal is often found on the 
«cum covering the surface of stagnant waters, but 
more frequently in the water found in the hollows 
of decayed trees after rain. 
In spring and summer nothing is more com- 
mon than to see the surface of the smaller kind of 
stagnant waters covered with a fine deep-green 
scum; and frequently the same kind of greenness 
is diffused throughout the whole body of the water: 
this green colour is entirely owing to an Animalcule 
of a genus called Cercaria^. I have myself de- 
scribed it under the name of Cercaria mutabilis or 
Changeable Cercaria, because a variety sometimes 
occurs of a red colour. The animal is of a length- 
ened oval shape, with a sliglitly lengthened tail, 
the body or middle part appearing as if filled 
with very numerous green spawn or ova, while 
the extremities are transparent. It occurs at 
this season of the year in almost every puddle. 
The red variety is far less common, and the ap- 
* Naturalist’s Miscellany, vol. iii. pi. 107, 
