THE LIFE OF THE SEA-SHORE. 137 
which form a very important part of the diet of such 
molluses as mussels and oysters. But before these two 
kinds of food can be utilised by animals, we must again 
have rocks on which the animals may settle down, for 
practically all shore animals which feed on microscopic 
particles are of relatively low organisation and sedentary 
habit, unable to resist the force of shore currents by their 
own activity, and therefore obliged to attach themselves 
firmly to some base. 
To recapitulate, shore animals are only found in number 
and variety in the vicinity of rocks, first because many of 
them are directly dependent upon the larger algze which can 
only flourish on rocks; second, because the microscopic algz 
and food particles in the water, which constitute the other 
great food supply, can only be utilised by sedentary animals, 
which must have a rock surface upon which to settle; third, 
because all other shore animals depend for food upon these 
two sets, and can only flourish where these are present in 
abundance. It must not be supposed that these three sets 
of animals are sharply marked off from one another, for 
limpets and periwinkles consume a large amount of diatoms 
as well as of the larger alge, and some carnivorous forms do, 
upon occasion, eat weed; but it may be helpful to think of 
the main sources of food in this way. By so doing it will 
be obvious that one peculiarity of distribution is easily ex- 
plained. If very many shore animals depend in whole or in 
part on particles carried down into the sea by rivers and 
streams, then such animals will tend to be more or less 
sharply confined to the neighbourhood of the river mouths. 
There are, in point of fact, numbers of animals’ which are 
rarely abundant except in such localities. Among these are 
many worms, some sea-squirts, mussels, oysters, and other 
bivalves, together with many more active animals which prey 
upon the sedentary forms. This fact is theoretically of 
much interest and importance, but practically it is not of 
very much value, for it was discovered long ago by the 
founders of our sea-coast towns and fishing villages, and 
most of these are placed near river mouths, so that in most 
cases those parts of the shore which are subjected to detailed 
investigation are likely to be near river mouths. Economists 
