ALLIES OF THE ACEPHALA 



209 



the atmosphere. Anyone who owns an aquarium contain- 

 ing snails may observe 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 

 admit the air. In aquariums 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. 



It is not uncommon to 

 see a pond snail crawling 

 upside down at the sur- 

 face of the water. The 

 explanation of this curi- 

 ous 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 gland con- 

 tracting 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 a water plant, or submerged object, and 

 forms a bed over which the animal moves. 



Fig. 112. Mucus threads of pond snail 

 in water. (Reduced) 



After Kew 



