108 INTRODUCTION TO ZOOLOGY chap. 



An examination of a section of the shell under the micro- 

 scope, shows that it consists of the following three distinct 

 layers : — 



(1) The thin, outer, horny layer mentioned above; 



(2) A thick, calcareous, opaque layer, formed of many 

 densely packed prisms of calcium carbonate, lying across the 

 thickness of the shell. This is known as the prismatic layer ; 



(3) An inner pearly, or " nacreous " layer, usually much 

 thinner than the prismatic layer, and formed of a number of 

 very thin calcareous layers lying one over the other. 



FiQ. 61. — The Common Pond Snail (Limno,ea stagnalis). 



A^ Seen from tl)e right side. B, Seen from below. (, Tentacle ; /, foot ; A, head ; 

 m, mouth ; r, respiratory aperture. 



Bo^y The shell protects the soft body of the snail. 



Structure which may be entirely withdrawn within it or 



(external), partly protruded beyond. 



The most conspicuous structure when the snail is fully 



extended, is the broad flat muscular foot on which the snail 



moves, with a smooth gliding motion, over any flat surface, 



always keeping the sole in close contact with the surface, but 



moving forward by a series of alternate expansions and 



contractions of the muscles of the foot which travel through 



it with a wave-like motion. 



Projecting above the foot in front, but attached to it 

 behind, is the head, a mere fleshy lobe, slightly indented in 

 front, but bearing on its under side the mouth, and on its upper 

 side a single pair of triangular " feelers " or tentacles. Just at 



