ORIGIN OF PEARLS. 501 



gressively adds to its weight and solidity. There is, moreover, a re- 

 markable 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 remain of a delicate white hue, and form the 

 well-known iridescent material usually distinguished by the name of 

 nacre, or mother-of-pearl. 



(1319.) 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. 



(1320.) Or pearls may owe their origin to another cause. It not 

 unfrequently happens that sharp angular substances, such as grains of 

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

 become imbedded in the delicate tissue of the mantle. Thus irritated, 

 the mantle throws out copiously the peculiar iridescent material which 

 it secretes, and with it coats over the cause of annoyance, wrapping it in 

 numerous concentric laminae of nacre, and thus forming the detached 

 and globular pearls so valuable in commerce. 



(1321.) One other circumstance connected with the growth of bivalve 

 shells requires explanation. From the earliest appearance of the shelly 

 valves until the period when the included mollusks arrive at their 

 mature size, the adductor muscle or muscles have been of necessity per- 

 petually changing their position, advancing gradually forward as the 

 enlargement of the shells was accomplished, so as to maintain in the 

 adult precisely the same relative situations as they originally did in the 

 young and as yet minute animal. Taking the Oyster for an example, it 

 is quite obvious that the adductor muscle, which at first was connected 

 with the thin and minute lamellaa forming the earliest shell, has, during 

 the entire growth of the animal, become further removed from the 

 hinge, and transferred from layer to layer as the shell increased in 

 thickness, till it has arrived at the position occupied by it in connexion 

 with the last-formed stratum that lines the interior of the ponderous 

 valves of the full-grown Oyster. The manner in which this progressive 

 advance of the adductor muscle is effected is not at first easily accounted 



