CONCHIFERA. 387 



texture. If exceedingly thick, the epidermic layer thus formed 

 becomes loose and shaggy, giving the shell a hirsute appearance ; 

 but, both in its structure and origin, such pilose investment has no 

 claim to be considered analogous to the hair of animals possessing 

 an epidermis properly so called. 



While the margin of the mantle is thus the sole agent in en- 

 larging the circumference of the shell, its growth in thickness is 

 accomplished by a secretion of a kind of calcareous varnish, derived 

 from the external surface of the mantle generally ; which, being 

 deposited layer by layer over the whole interior of the previously 

 existing shell, progressively adds to its weight and solidity. There 

 is, moreover, a remarkable difference between the character of the 

 material secreted by the marginal fringe, and that furnished by the 

 general surface of the pallial membrane ; the former we have found 

 to be more or less coloured by glands appointed for the purpose, 

 situated in the circumference of the mantle ; but as these glands do 

 not exist elsewhere, no colouring matter is ever mixed with the layers 

 that increase the thickness of the shell, so that the latter always re- 

 main of a delicate white hue, and form the well-known iridescent ma- 

 terial usually distinguished by the name of nacre, or mother of pearl. 



(422.) Local irritation of various kinds is found to stimulate the 

 mantle to increased action, so as to cause the pearly matter to be 

 secreted more abundantly at the part irritated. Thus there are 

 various minute boring annelidans that, in the exercise of their usual 

 habits, perforate the shells of oysters, and penetrate even to the 

 soft parts of their bodies. Stimulated by the presence of these 

 intruders, the mantle beneath the place attacked secretes nacre in 

 inordinate quantities to repair the injured portion of the shell, and 

 prominent nuclei are soon formed, which, enlarging by the addition 

 of continually added layers of nacreous matter, become so many 

 pearls adherent to the interior of the shelly valves. 



Or pearls may owe their origin to another cause : It not unfre- 

 quently happens that sharp angular substances, such as grains of 

 sand or fragments of stone, are conveyed between the valves, and 

 become embedded in the delicate tissue of the mantle. Thus irri- 

 tated, the mantle throws out copiously the peculiar iridescent mate- 

 rial which it secretes, and with it coats over the cause of annoyance, 

 wrapping it in numerous concentric laminae of nacre, and thus form- 

 ing the detached and globular pearls so valuable in commerce. 



(423.) One other circumstance connected with the growth of 

 bivalve shells requires explanation. From the earliest appearance 



