Gastropoda. 131 



the body, near the aperture of the shell, is a space called 

 the mantle cavity. In this space lie the gills, but in many 

 cases but one gill is developed. 



Air-breathing Gastropods. In land snails, slugs, and 

 numerous fresh-water snails, the mantle chamber becomes 

 lore shut in, leaving a narrow opening, which is near the 

 right side in right-hand snails, and on the left in the left- 

 hand snails. Through this opening, which is kept closed 

 most of the time, air is taken into the cavity, which acts as a 

 lung. The blood circulates around the walls of the lung 

 cavity, and is thus brought close to the air, so that an inter- 

 change can take place between the two. 



The Land Snails. These are abundant in damp woods, 

 especially in limestone regions. Their shells are usually 

 thin. Land snails have two pairs of tentacles, with eyes at 

 the tips of the upper or longer pair. The eyes can be 

 pulled in, the tip disappearing first, as when in pulling off 

 a glove the tip of the glove finger sticks to the end of the 

 finger. If the tip of one of these tentacles is cut off, it will 

 be reproduced, and it is said that this has been done twenty 

 times in succession. 



Land snails usually have no operculums ; but at the ap- 

 proach of winter, or of a period of drouth, they bury them- 

 selves in the ground, and pull the body in until the foot is 

 even with the edge of the aperture. A layer of mucus is 

 secreted which completely closes the aperture. In some 

 cases limy material is added, and, in any case, the covering 

 soon hardens. Sometimes the snail then withdraws still 

 farther, and makes another such barrier, or even several. 

 In the spring, or at the return of moisture, the temporary 

 door is cast off, and the snail resumes its activity. Snails 

 have great vitality, and have been known to survive in this 

 shut-in condition for six years without food. 



