1NFUSORY WORMS* 549 



is remarkable for its strange power of reviviscence, or restoration to 

 life and motion, after being dried for 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 hol- 

 lows of decayed trees after rain. 



In spring and summer, nothing is more common 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 dif- 

 fused throughout the whole body of the water : this green colour 

 is entirely owing to an animalcule of a genus called cercaria*. It 

 has of late been described under the name of cercaria mutabilis, or 

 changeable cercaria, because a variety sometimes occurs of a red 

 colour. The animal is of a lengthened 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 trans- 

 parent. It occurs at this season of the year in almost every puddle. 

 The red variety is far less common, and the appearance which it 

 exhibits is such as to alarm the superstitious with the idea of the 

 water being changed into blood ; a panic of which numerous in- 

 stances have been adduced by authors; and which is the more excus- 

 able in those who are ignorant of the cause, as the animalcules are 

 so very small as to be utterly imperceptible, except to an uncom- 

 monly acute eye, without the assistance of the microscope ; so that 

 even taking up, and examining it, affords no satisfactory elucidation 

 to the vulgar. Not unfrequently we have seen the whole sur- 

 face of a large pond thus covered with this animalcule, of which 

 there was not the least appearance the preceding day. It should 

 be observed that some other animals, and particularly some small 

 insects of the genus monoculus, have occasionally produced a simi- 

 lar appearance : but in that case the demonstration becomes easy; 

 since every one, on taking up the water, perceives the red insects. 

 We are assured by Swammerdam, that the whole city of Leyden 

 was one morning in a state of consternation, on discovering that the 

 waters of that place were apparently changed into blood ; but the 

 philosopher soon had the satisfaction of undeceiving the people, by 

 demonstrating to them the real cause. 



Among the most remarkable of the animalcular tribe may be 



* Naturalist's Miscellany, vol. Hi. pi. 107. 



