THE LIFE OF THE SEA-SHORE. 141 
is a good spring tide and a touch of east in the wind, and 
you will understand that the shore animals must take 
as their first motto “Grip fast.” Follow the retreating 
waves backwards under a hot summer sun and you will 
realise in the second place the necessity for a hard coat 
or shell, or for some other protection against the risk of 
drying up. A typical shore form must therefore first 
protect itself from the force of the breakers, and, second, 
against the strong summer sun or the frosts of winter when 
the tide ebbs. There are many ways in which these two 
ends may be attained, but perhaps the simplest is that 
exemplified in the common limpet (Patella vulgata). The 
limpets are usually the most conspicuous of shore animals 
at low tide—tightly fixed to the surface of the rock they 
seem indifferent to the absence of their natural element or 
to any variation in temperature. But if you knock a 
limpet off the rock on a summer’s day, or look at those 
which the strong bills of the birds have knocked off, you 
will notice that while a fresh limpet is always abundantly 
moist on its under surface, one which has been exposed 
even for a short period is hard and dried. You will notice, 
also, that the under surface consists almost entirely of 
a great muscle, called the foot, by means of which the 
limpet clings to the rock. The limpet, therefore, solves 
its two problems in the simplest possible fashion. When 
the tide rises it wanders forth in search of food, when it 
ebbs it returns to its own particular niche in the rock. In 
some way which is not well understood, it hollows out 
the rock so that the shell fits closely into a depression. 
The large foot and the conical -shell give great clinging 
power, while the density of the shell prevents loss of water 
by evaporation through it. Though the sun beat down 
or the breakers dash over the rock, the limpet is safe. 
From the birds, however, it is not safe. They dislodge 
it by a sudden blow of the beak, and turning over the 
shell, devour its inhabitant at their leisure. Against such 
attacks the limpet can do nothing. Compare with it 
the periwinkles and dog whelks also so abundant on the 
shore rocks. They have nothing like the clinging power 
of the limpet, and on any sudden alarm prefer to drop from 
VOL. I. 12 
