SCALLOP. 67 



zation, while the latter may be instanced as affording 

 modifications of structure which adapt it to the pecu- 

 liar locality to which it is confined. 



On opening the valves of a living Scallop we perceive, 

 within the margin of the shell, a soft membrane, which 

 lines the whole of the inner surface, and encloses the 

 body of the animal as in a cloak, open in front through 

 the centre ; so that a curtain, fringed round the edge 

 with innumerable slender filaments, hangs from each 

 valve of the shell. This membranous envelope, which 

 is called the mantle, exists, though under many modifi- 

 cations, in all the Mollusca, and indeed is one of their 

 most essential parts. It is by means of this organ that 

 all the shell-coated tribes cover themselves with the 

 beautiful shells which are objects of so general admi- 

 ration. The thickened margin of the mantle is fur- 

 nished with glands which secrete both colouring-matter 

 and carbonate of lime. From the latter material, de- 

 posited in cellular substance derived from the animal, 

 the shell is gradually formed by constant additions to 

 its margin ; while the colouring-matter, poured in at 

 the same time, gives to the outer surface all the peculiar 

 markings which characterize each kind. The outer 

 coat of the shell is therefore entirely the work of the 

 margin of the mantle. Its increase in thickness is an 

 after process, effected by the general surface of this 

 organ, which throws off layers of pearly substance, 

 and adds them continually, one after another, to the 

 inner surface of the shell. Thus, as the shell increases 

 in size, its walls grow in thickness. In the Scallop, 

 among the fringing processes of the margin, are found 



