WATER SNAILS. 183 
sustenance from the snail, but feed on animal- 
cules. 
From experiments made by Mr. Jennings and 
Dr. Ball, some of the water-snails appear to have 
the power of causing a peculiar sensation when 
placed on the tongue. The experimentalists 
operated upon the common pond-snail (Limnea 
peregra) ; they allowed the foot of the animal to 
remain for a few minutes on the tongue, when 
the sensation was felt, varying in intensity ac- 
cording to the size of the animal, and the length 
of time it is allowed to remain. The sensation, 
though not decidedly painful, is yet rather dis- 
agreeable whilst it continues, frequently lasting 
from one to two hours, bemg exerted with 
greater energy during warm than cold weather. 
The same painful sensation was experienced by 
the contact of the river hmpet (Ancylus) with 
the tongue. A few experiments were tried to 
ascertain whether the power arose from an acid 
secretion, capable of beimg emitted at pleasure 
by the animal, but so far without success. Have 
the pond-snails stinging or urticating organs of 
the nature of those which have been recently 
observed in the marine slugs (Holide) ? ° 
The family is represented in Great Britain by 
four genera, which may be distinguished as 
follows :— 
