Introductory 2 5 



or other layers of this shell, as is proved by the repair 

 of breakages. Many an adult shell bears signs of 

 earlier accidents, the fractures being always evident 

 from the fact that the colour patterns of the original 

 are not reproduced. It is only the edge of the mantle 

 that can produce the colour and pattern, and this is 

 the part that first makes all the additions to a grow- 

 ing shell to allow room for the growth of the animal, 

 w^hilst the other parts of the mantle merely strengthen 

 this first layer by backing and thickening it. The 

 upper layer therefore carries the pattern and colour, 

 whilst the lowest layer is usually white and highly 

 polished. Outside all there is a kind of animal 

 varnish, protecting the shell from many injuries, and 

 this is known sometimes as the epidermis, but more 

 correctly as the periostracum. Sometimes the shelly 

 matter assumes the crystalline condition, the carbonate 

 of lime having taken the form of calcite or aragonite. 

 The statement made above that the fluid shell-matter 

 is poured out and moulded upon the external surface 

 of the mantle is the modern view generally held, but 

 some authorities agree with the late Dr. AV. B. 

 Carpenter, who held that it was secreted in the cells 

 and interstices of the outer membrane of the mantle, 

 which was afterwards separated. 



The Snail breathes air, but its " lung " is merely a 

 cavity opening directly on the surface of the animal 

 and closed at will, in a manner corresponding to the 

 opening and closing of our lips. Its inner walls are 

 lined with a fine network of vessels through which 

 the blood courses, absorbing oxygen from the moist 

 air and giving off carbon. This simple form of lung 

 is referred to as the Pulmonary Sac, and its external 



