FORMATION OF SHELL. 499 



of within the shell, but its action in antagonizing the adductor muscles 

 is still equally efficacious. 



(1310.) We must, in the next place, solicit the attention of the reader 

 to a very important subject connected with the economy of this class 

 of Mollusks, viz. the growth and formation of their shells. Infinitely 

 diversified are the forms presented by their testaceous valves, and 

 equally various the colours which not unfrequently adorn their external 

 surfaces. Some exhibit a beauty and delicacy of sculpture of a most 

 exquisite character ; others, covered with large spines, or festoons of 

 calcareous plates, puzzle the beholder to comprehend how the growth of 

 such parts, in the situations which they occupy, can be effected with so 

 much regularity of arrangement. The shells themselves are permeated 

 by no vessels, and as incapable of expansion by any internal power as 

 the rocks to which they are not uncommonly attached ; so that the young 

 naturalist is necessarily at a loss to conceive either the mode of their 

 formation, or the origin of all the gaudy tints and external decorations 

 that render them the ornaments of our cabinets. 



(1311.) The simple apparatus by means of whiqh shells are con- 

 structed is the external membranous layer that invests the body of the 

 mollusk the mantle, as it has been termed ; and, whatever the form 

 of the shell, it owes its origin entirely to this delicate organ. 



(1312.) In order to simplify as much as possible our description of 

 the process whereby the shell is formed, it will be necessary to consider 

 it under two points of view : first, as relates to the enlargement of the 

 valves in length and breadth ; and secondly, as regards their increase 

 in thickness, very different parts of the mantle being employed in the 

 attainment of these two ends. 



(1313.) It is the circumference, or thickened margin, of the mantle 

 alone which provides for the increase of the shell in superficial extent. 

 On examining this part (fig. 247, h ; fig. 248, e), it is found to be of 

 a glandular character, and moreover not unfrequently provided with a 

 delicate and highly sensitive fringe of minute tentacula. Considered 

 more attentively, it is seen to contain in its substance patches of dif- 

 ferent colours, corresponding both in tint and relative position with 

 those that decorate the exterior of the shell. 



(1314.) When the animal is engaged in increasing the dimensions of 

 its abode, the margin of the mantle is protruded, and firmly adherent 

 all round to the circumference of the valve with which it corresponds. 

 Thus circumstanced, it secretes calcareous matter, and deposits it in a 

 soft state upon the extreme edge of the shell, where the secretion 

 hardens and becomes converted into a layer of solid testaceous sub- 

 stance. At intervals this process is repeated, and every newly-formed 

 layer enlarges the diameter of the valve. The concentric strata thus 

 deposited remain distinguishable externally ; and thus the lines of growth 

 marking the progressive increase of size may easily be traced (fig. 250). 



2x2 



