110 INTRODUCTION TO ZOOLOGY CHAP. 



To renew the air in this lung, the snail has to come 

 frequently to the surface of the water ; turning over, so as to 

 bring the respiratory aperture just to the surface, it opens 

 it with a little audible pop, and then causes the expulsion 

 of the impure air and inrush of fresh air, by the alternate 

 raising and flattening of the floor of the chamber, which is 

 formed of the muscular dorsal wall of the body. The 

 aperture may be kept open for a minute or two, but is 

 always closed again before the snail descends in the 

 water. 



The usual method of movement of a snail is 

 ' the slow gliding motion over a surface described 

 above, but the " lung " is sometimes made use of to cause 

 rapid descent in the water when the snail is irritated. 

 Normally, when the lung is full of air, the snail floats in the 

 water, shell uppermost, but the air may be suddenly expelled 

 with force from the mantle chamber, owing largely to the 

 sudden withdrawal of the body into the shell, and the con- 

 sequent upward pressure on the floor of the cavity ; when 

 this occurs the body sinks rapidly to the bottom of the 

 water. After such a movement, the snail has soon to come 

 to the surface again to breathe, climbing laboriously up a 

 plant or the sides of the tank in which it is living. It will 

 then frequently move across the surface of the water with 

 its shell hanging downwards, and with the margin of the sole 

 of the foot on a level with the water, the rest of its surface 

 being slightly depressed. It is supported in this position by 

 the tension between the margin of the foot and the surface 

 film of the water. 



When the snail is low down in the tank it can, if its 

 lung is full of air, rise rapidly to the surface, merely by 

 letting go of the plant to which it is clinging. This is an 

 advantage in enabling it to stay below whilst feeding, until 

 the need for fresh air is urgent, when it can in this way 

 rise very rapidly to the surface to renew its supply. 



Some species of Limnaea have been observed to aid their 

 motion vertically in the water, by forming, from a mucous 

 secretion of the foot, a delicate rope of mucus, up and down 

 which they travel. This " rope " may be fastened at first to 

 some plant or stone in the water, then stretched out as 

 the snail floats up, and again attached at the surface ; or, in 



