active little things, and are usually so
different from their parents as to be unrecognisable. Some swim, or
frisk about, and travel even long distances in search of suitable
quarters to settle in. Others float on the surface, and are driven where
the winds and currents list. Some, like mussels, are distributed all
over the world, others again are found, perhaps, on one rock, or on one
small sandbank in a large district. Many shells are rare, because we do
not know where to look for them; but if we know and can find their food,
we will find the shellfish not far away. Some change their shape so much
that, as they age, they have to dissolve all the partitions made in
their youth in the shell. The eggs of some are scattered on the surface
of the water, while the eggs of others are hatched by the mother before
being turned adrift.

Marine shellfish live in all kinds of places below high water mark; and
some of the semi-amphibious ones thrive even above ordinary high water
mark, where for days at a time nothing but the tops of the waves could
reach them. They are found on seaweed and on rocks, and on sand or
mud-banks; but especially in places near rocks on marine grass banks
bare at low spring tides. Some live on the surface of the water, some
burrow in sand or mud, and some bore holes for themselves in the softer
rocks. Some live in deep water; but the better coloured shells are found
near low water mark, or in shallow water; for light is as necessary to
the perfecting of colour in shells as in flowers. Shells that have grown
in a harbour are more fragile than those grown in the ocean, and are
usually less brilliant in colour, as harbour water is not as clean as
ocean water. The colour of shells (as of insects) depends largely on
environment, and is only one, and by no means the most reliable, method
of deciding the species. An expert can at a glance tell whether a given
shell has come from shallow or deep water, and whether from an exposed
or sheltered spot. Most shellfish move about a great deal, and migrate
into deeper water in summer; and on bright clear days retire into dark
corners amongst, and even under, stones. On a dull day a collector is
frequently more successful than on a bright, sunny day; and in spring or
early summer the best hauls of live shells can be made. Nearly all
shells have an epidermis, or outer skin. In some this is very apparent,
as in the Lotorium olearium (Plate V., Fig. 1), or the Solenomya