MOLLUSCA 



245 



may be obtained by cutting .off the head of the snail and 

 boiling it in caustic potash solution, which dissolves 

 away everything except the jaw and lingual membrane. 

 The surface of the membrane, examined under the com- 

 pound microscope, is seen to be covered with innumer- 

 able delicate teeth, arranged in rows. These teeth are 

 different in different kinds of mollusks, and are of great 

 value for classification. Around the edge of the shell, 

 in front, will be seen a soft fold, which is the margin of 

 the mantle. This is the organ which secretes the shell, 

 adding always material around the aperture, until the 

 animal is mature, when the work is usually finished by 

 addition of a thickened edge, the lip. On the right- Breathing 

 hand side, when the snail is active, there appears an apparatus 

 opening, which admits air to the lung. The lung is a 

 simple moist sac, the walls of which are richly supplied 

 with blood vessels. The lung-breathing snails are 

 called pulmonates, and include many terrestrial and 



From "Animate Creation' 

 FIG. 62. A large African land snail, Achatina, about natural size. 



