ALLIES OF THE PELECYPODA: MOLLUSCA 179 



The two tentacles are organs of touch. The eyes lie one 

 in front of either tentacle at its base. On the right side, 

 near the head, are the single openings of the intestine, the 

 nephridium, and the sexual glands. Also on the right side, 

 and partly beneath the edge of the shell, is an aperture 

 which opens and closes over a small hollow space in the 

 body. This space is the so-called "lung" of the pond-snail. 

 The lung is adapted to using the oxygen of the atmosphere. 

 Any one who owns an aquarium containing snails may ob- 

 serve them crawling up the side of the vessel on their way 

 to the surface. When they arrive there they remain for some 

 time, opening and closing the lung to the air. In aquaria 

 which have a supply of water-plants growing in them, the 

 pond-snail does not appear to go to the surface so often. It 

 is well known that plants give off oxygen. Some of this 

 oxygen held in the water may pass through the snail's skin, 

 which is thin and soft. The waste carbon dioxide of the snail 

 is discharged into the water, and may be used by water- 

 plants as raw food-material. 



It is not uncommon to see a pond-snail crawling upside 

 down at the surface of the water. The explanation of this 

 curious phenomenon is the same as the explanation of how 

 they can get along on any kind of a surface. Just below the 

 mouth is the opening of a gland, which extends through the 

 middle of the foot near the lower surface. If we look closely 

 at the inverted snail, we can see the wall of this foot-gland 

 contracting with a wave-like movement in the act of sending 

 the secreted mucus (slime) forward to be poured out at the 

 opening. The mucus spreads out a short distance on the 

 water, or blade of water-plant, or submerged pebble, and 

 forms a bed over which the animal moves. 



When the foot is extended in locomotion the pond-snail 

 weighs less than an equal volume of water. If the animal 

 releases its hold on an object at the bottom, it floats to the 



