THEIR ORGANIZATION. 143 



consists of two cylinders, one within the other; the outer 

 of which serves for the attachment of the motor muscles 

 and the general protection of the organ, while the inner, 

 opening near the extremity with a longitudinal mouth, 

 armed with two strong cartilaginous lips, encloses the 

 tongue and a great part of the oesophagus. The tongue 

 is armed with sharp spines, and, acting in concert with 

 the hard lips, which can be opened or shut, or strongly 

 pressed together, it forms a sort of rasp or auger, by 

 which very hard substances are rapidly perforated ; and 

 then the tongue being protruded, the hooked spines 

 with which it is armed are admirably fitted for the col- 

 lection of food. The mode in which the shells of Gas- 

 teropoda are formed is very similar to what takes place 

 among bivalve shells. These beautifully-painted struc- 

 tures are secreted by the glandular margin of the 

 mantle, or soft skin, which clothes the upper part of the 

 body of the Mollusc; and their form depends on the 

 shape of the body they are destined to cover, while the 

 outline of the border is alike regulated by that of the 

 mantle. In the border of the mantle are placed the 

 glands through which colouring-matter is added to the 

 lime of which the shell consists, and here also the whole 

 of the outer coat of the shell is formed by constant an- 

 nual additions to the lip. The after-growth of the shell 

 in thickness, is provided for by secretions almost always 

 colourless, from the general surface of the mantle. These 

 are supplied in thin layers, one over the other, at stated 

 periods, so that the older a shell is, the thicker will be 

 the substance. In most of the shell-coated Gasteropoda 

 the mantle is concealed by the shell, or its margin only 



