THE POND SNAIL. 



409 



and stout footstalks, and the enormously long tentacles are placed just in front of the 

 eyes. At the first glance the creature appears to have four tentacles ; but, on a closer 

 examination, the front pair are seen to be merely developments of the muzzle. In one 

 respect, the Ampullaria seems to be a connecting link between the gill-bearing and 

 lung-bearing molluscs, being said by high authorities to possess a pulmonic or lung 

 sac, in addition to its gills. 



ON the left hand of the illustration the common POND-SNAIL, or LIMN^EA, is shown 

 in the act of climbing up the stem of a water-plant. In all the members of this family 

 the shell is thin, and sufficiently capacious to contain the entire animal when it desires 



RAM'S-HORN APPLE-SNAII Ampullaria cornu-arietls. 



WATER-SNAIL. Llmneea stagnalls. POUCH-SHELL. Physa hypnorum. 



COMMON PLANORBIS. Planorbls cornea. 

 STAIRCASE APPLE-SttELL.-AmpuIlarla scalarls. RIVER LIMPET. Ancylus lacusMs. 



to withdraw itself into its home. The aperture is simply rounded, without notches or 

 ridges, and the lip is sharp. 



The Pond-snail may be found in almost any fresh water, and, if carefully watched, 

 proves to be quite an interesting creature. It can creep with tolerable rapidity after 

 the usual manner of snails, and has besides a curious method of progression without 

 making any exertion of its own. In streams, when the animal has a mind to change 

 its locality without needing to exert itself, it achieves the task of converting the 

 journey into a voyage, and its foot into a boat. This transformation is soon effected, 

 the animal first crawling up some plant that projects out of the water, reversing its posi- 

 tion, so that the shell lies undermost, and then hollowing the foot so as to form it into a 



