BIVALVE MOLLUSCA. 365. 
grina margaritifera ; being the same secretion which in the pearl has 
assumed the globular form: in one state it is deposited as nacre on 
the walls of the bivalve, in the other as a pearl in the fleshy interior 
of the animal. This nacre is therefore at once a calcareous and a 
horny substance, which the animal secretes, and which it attaches to 
the interior walls of the shell during the several periods of its develop- 
ment. Pearls are formed of the same substance, only in place of 
being deposited upon the valves in beds, the material is condensed 
and agglomerated in small spheroids, which develop themselves 
either on the surface of the valves or in the fleshy part of the mollusc. 
(Fig. 164.—Meleagrina margaritifera (Linnzus). 
Between nacre and pearls, therefore, there is only the difference of 
the form of deposition. Fig. 164 represents the pearl oyster with 
calcareous concretions in various states of progress. 
The finest pearls—solidified drops of dew, as the Orientals term 
them in the language of poetry—are secretions of nacrous material 
supposed to be spread over foreign bodies which have accidentally 
got beneath the mantle of the mollusc. The matter, in place of 
being spread over the surface of the valves in their beds, is condensed 
either on the centre of the valves or in the interior of the organ, and 
forms a more or less rounded body. The pearls, when deposited on 
the valves, are generally adherent ; those which originate in the body 
of the animal are always free. Generally we find some small foreign 
body in their centre which has served as a nucleus to the concretion, 
the body being perhaps a sterile egg of the mollusc, the egg of a fish, 
