586 APPENDIX 



characterises mother-of-pearl, a lustre which is exhibited by all substances having the same 

 structure, that is to say, all substances which are built up of transparent, overlapping 

 laminas. 



The laminae which form the nacre or mother-of-pearl layer of the shell do not lie in 

 planes parallel to the surface of the shell, but are always more or less bent and curved. 

 Their edges intersect the inner surface of the shell, and produce a very fine striation, which 

 can sometimes be seen with a simple lens, but which usually requires the help of the 

 compound microscope. These stria? or furrows take zig-zag or quite irregular courses, 

 sometimes indeed forming small closed rings ; the distance between them averages 

 -STfisu inch, but varies within small limits. It is to these striae that the play of colours so 

 characteristic of mother-of-pearl is due, while the peculiar lustre of the substance is due to 

 its laminated structure, as has been already explained. This play of colours, then, is quite 

 independent of the presence of pigment of any kind. It is due to purely physical causes, 

 the fine strias so acting upon the rays of ordinary daylight or candle-light as to split them 

 up into their coloured constituents ; these reach the eye singly, and so produce the 

 sensation of colour. That this is the true explanation is proved by the fact that the same 

 play of colours is to be seen upon sealing-wax after it has been pressed upon the natural 

 surface of the mother-of-pearl layer, or, better still, upon a section of this layer cut obliquely 

 to the surface, and has thus received an impression of the strias. 



The body of a bivalve mollusc is produced into two lateral flaps, the so-called mantle, 

 which lie in immediate contact, with the inner surface of the shell. The different parts of 

 the shell of the mollusc are secreted by the mantle, the cells of which have the power of 

 separating out the calcium carbonate dissolved in the water in which the animal lives. 

 Different areas of the mantle have somewhat different functions in this respect : thus the 

 edge of the mantle secretes the periostracum, the so-called " epidermis " ; the outer surface 

 lays down the calcareous lamina;, of which the nacre is composed ; while the prismatic layer, 

 between the periostracum and the mother-of-pearl or nacre, is secreted by a narrow zone 

 round the margin of the mantle. 



This, then, is the process of shell-formation which goes on under ordinary conditions, 

 and so long as these obtain no pearls are produced. The formation of a pearl is the 

 response of the mollusc to the stimulus afforded by a local irritation of the mantle. Being 

 unable to eject the cause of the irritation, the mollusc obtains relief by enveloping it with 

 a deposit of mother-of-pearl substance. The rounded aggregation of mother-of-pearl 

 substance so produced is then known as a pearl. 



That the formation of pearls is an abnormal occurrence in the life of a mollusc is 

 shown by the fact that among the pearl-forming molluscs, only about one in thirty or forty 

 is found to contain pearls. Moreover, the observations of pearl-fishers all point in the 

 same direction ; for they state that there is little prospect of finding pearls in a well-formed, 

 normal shell, and that the shells most likely to contain pearls are those which are irregular 

 and distorted in shape, and which bear evidence of having been attacked by some boring 

 parasite. That the formation of pearls is caused by some disturbance of the normal 

 conditions of life may, therefore, be regarded as a well-established fact ; but the exact 

 causes which bring about this secretion are not in all cases satisfactorily explained. 



The nucleus of many pearls is a tiny grain of sand, and in such cases it is obvious that 

 this foreign body set up an irritation of the mantle and caused an abnormal secretion of 

 calcium carbonate, in the same way as a particle of dust in the eye causes a copious flow of 

 water in that organ, or as the presence of a trichina (flesh-worm) in a muscle-fibre causes 

 the latter to secrete a calcareous cyst around the intruder. Many observers, following 



