6o THE BOOK OF NATURE STUDY 



The outside layer alone is coloured ; it is of horny matter (conchi- 

 olin), and is termed the periostracum ; the middle and innermost 

 layers are both white, in consequence of the abundance of calcareous 

 matter which they contain, but have a different " grain." The 

 middle layer is made up of a number of calcareous prisms, hence 

 it is known as the " prismatic layer/' but the innermost is of 

 alternating sheets of pure conchiolin and of calcified conchiolin. 

 This last arrangement produces the characteristic iridescence of 

 the innermost layer, which is therefore known as the " mother- 

 of-pearl " or " nacreous " layer. The periostracum is formed only 

 at the margin of the mantle, and if injured elsewhere cannot be 

 repaired ; along the margin, where freshly formed, it exists as 

 the sole layer, and is quite flexible in the absence of all calcareous 

 " backing/' The nacreous layer, on the other hand, is continu- 

 ously formed by the whole surface of the mantle, and if the shell 

 be cracked or injured in any way it is this layer that is used to 

 repair the rent. Pearls, which are often found, though of inferior 

 quality, in mussels are made of this material. The animal may 

 be killed instantly by plunging into boiling water. In order to 

 see the soft parts of the animal it is necessary to force open the 

 shell- valves. This may be done by inserting a blunt flat instru- 

 ment, e.g. the handle of a scalpel or of a pair of forceps, edgeways 

 between the valves, and then turning it through a right angle. 

 The valves will thus be thrust apart sufficiently far to allow in- 

 struments to be inserted. The next step is to separate one valve 

 from the soft mantle-flap which lines and is attached to it. This 

 should be done by passing the handle of another scalpel between 

 the two structures and running it all round the margin so as to 

 sever the pallial muscles. The handle should then be used to 

 ascertain the position of the two adductor muscles, which should 

 then be cut through as close to the shell as possible with the 

 scalpel blade. The valve will then spring open and may be de- 

 tached. It is then an easy matter to remove the second valve 

 in the same manner. The soft animal thus extracted should be 

 placed in a dish of water for examination, and may possibly 

 require to be washed under the tap to remove the slime if this 

 be abundant. 



Covering the whole of the outside is the mantle, to which 



