It follows that the shore must be the scene of a keen struggle for
existence--which includes all the answers-back that living creatures
make to environing difficulties and limitations. There is struggle for
food, accentuated by the fact that small items tend to be swept away by
the outgoing tide or to sink down the slope to deep water. Apart from
direct competition, e.g. between hungry hermit-crabs, it often involves
hard work to get a meal. This is true even of apparently sluggish
creatures. Thus the Crumb-of-Bread Sponge, or any other seashore sponge,
has to lash large quantities of water through the intricate canal system
of its body before it can get a sufficient supply of the microscopic
organisms and organic particles on which it feeds. An index of the
intensity of the struggle for food is afforded by the nutritive chains
which bind animals together. The shore is almost noisy with the
conjugation of the verb to eat in its many tenses. One pound of rock-cod
requires for its formation ten pounds of whelk; one pound of whelk
requires ten pounds of sea-worms; and one pound of worms requires ten
pounds of sea-dust. Such is the circulation of matter, ever passing from
one embodiment or incarnation to another.
Besides struggle for food there is struggle for foothold and for fresh
air, struggle against the scouring tide and against the pounding
breakers. The risk of dislodgment is often great and the fracture of
limbs is a common accident. Of kinds of armour--the sea-urchin's
hedgehog-like test, the crab's shard, the limpet's shell--there is great
variety, surpassed only by that of weapons--the sea-anemone's
stinging-cells, the sea-urchin's snapping-blades, the hermit-crab's
forceps, the grappling tentacles and parrot's-beak jaws of the octopus.