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. 



FIG. 61. The Common Poud Snail (Limnaea staynalis}. 



A, Seen from the right side. B, Seen from below, t, Tentacle ; /, foot ; h, head ; 

 ra, mouth ; r, respiratory aperture. 



Body The shell protects the soft body of the snail, 



Structure which may be entirely withdrawn within it or 

 (external). p art iy 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 



