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- Animal is often found on the 

 scum 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 slightly 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. 



