INTRODUCTORY CHAPTER. 



9 



Having thus given a short general description of the 

 animals inhabiting shells, the next object of inquiry is 

 the formation of shell itself, which is composed of car- 

 bonate of lime (the mollusc obtaining this substance 

 from its food — seaweeds, or other shells), and a small 

 portion of animal matter. Forbes says, that those dis- 

 tricts which are destitute of lime have also no shelled 

 Mollusca. The mollusc has a small shell attached to 

 it, even before the egg is hatched, and this is generally 

 of a pale horn colour, destitute of markings, and al- 

 most transparent ; when, therefore, it remains attached 

 to the apex of the spire in adult shells, it may be 

 easily distinguished by its appearance, from the additions 

 made afterwards. As the animal increases in bulk, it 

 finds the shell is not sufficiently large to cover it, and 

 it then adds to its dimensions by the aid of its mantle. 

 The animal, as before mentioned, has the power of causing 

 a liquid matter to exude from this portion of its body, 

 which is composed partly of membrane, and partly of 

 carbonate of lime ; this, on exposure to the air or water, 

 becomes shell, and the mollusc makes the addition to its 

 original habitation by protruding the mantle beyond the 

 edge, and suffering it in that manner to remain exposed : 

 in a short time it is found to be covered with a fluid 

 which has been secreted, and this gradually and slowly 

 assumes consistence, fresh layers being added within, each 

 projecting a little farther at the edge, and thus the new 

 formation becomes similar to the shell already in exist- 

 ence ; by this means are formed what are called lines of 

 growth. This operation is repeated at intervals, as the 

 animal increases, so that by this beautiful contrivance the 

 shell and its inhabitant are always adapted to each other, 

 and preserve their relative proportions. 



As the new portion of shell is always similar in shape, 

 and, in fact, takes its form from that part of the mantle 



