STRUCTURE OF BIVALVES. 287 



and there can be no doubt that, were we fully 

 aware of the habits, the. modes of life, the degree 

 of exposure to danger, or other peculiarities of 

 each of the bivalves, we should perceive that the 

 special differences even in this apparently unim- 

 portant part of their structure, so far from being 

 accidental, have been expressly intended for a 

 certain purpose. 



On opening the cockle, we perceive that the 

 whole of the inner surface of the two valves, 

 or shells, is lined with a smooth and delicate 

 membrane, or skin. This membrane, because it 

 encloses the body of the animal, is termed the 

 mantle. It is found in all the molluscs, although 

 subject to various modifications. It is, in fact, 

 an essential part of their structure, and among 

 the marvels of creative design there are few 

 things more wonderful than the office which this 

 apparently simple membrane performs. The 

 mantle is an apparatus adapted to form the shell 

 by which the mollusc is covered, and to deposit 

 the colouring matter by which it is adorned. 

 Simple as its structure appears to be, the manner 

 in which it is fitted to effect this object, far 

 exceeds the highest efforts of human ingenuity. 

 It is composed of minute cells, differing in size, 

 shape, and arrangement, in different species of 

 molluscs, and containing calcareous matter se- 

 creted from the fluids of the animal. The edges 

 of the membrane are occupied in adding con- 

 tinually to the edge of the shell, as its occupant 

 increases in size, and the inner portions of it are, 



