io6 LIFE AND HER CHILDREN. 
mantle lays down its own separate valve, and this is 
the case with all those mollusca which have no 
heads ; they all grow bivalve, or two-valved, shells, 
while those which have heads, such as periwinkles, 
snails, and whelks, have their mantle all in one piece, 
and consequently grow single or univalve shells. 
Nor is this all, for the shape, colour, and pecu- 
liarities of all the different shells come from pecu- 
liarities of the mantle. If this is crumpled at the 
edge or drawn out in horn-like folds, then the shell 
will have a crumpled form like the scallop, or horns 
like the murex, while the sunlight falling upon the 
mantle seems to help it in forming the bright pig- 
ment with which it paints its home, so that shallow- 
water shells and those of the tropics are more brightly 
coloured than those from the deep sea or from dull 
climates. Again in the inside of the shell, if the 
mantle leaves a smooth layer this will be white, but 
when the film is crumpled in very fine folds, these 
reflect the light in such a way as to give the beautiful 
colours known as mother-of-pearl ; while, if the mantle 
be irritated at any point, it will form in the oyster 
or the mussel a little bead of lime afterwards to be 
increased into a pearl. 
And now with this picture in your mind of the 
mantle at work, visit any good collection of shells, 
such as that at the British Museum, and look at the 
giant Strombs and Volutes of the Indian Ocean ; the 
Pinnas from the Mediterranean, half a yard long, 
with their erect curled scales ; the prickly Murex 
with its delicate pink-tinted lining ; and the gorgeous 
purple Mussels. Notice the rainbow-coloured cham- 
bers of the Nautilus, the pearly lining of the Haliotis, 
