There are practically no rocks on our Southeastern coast, so that we must imagine ourselves now somewhere in New England—let us say on the southern shore of Rhode Island. All along the north side of Long Island Sound, about Buzzards and Narragansett bays, and then from Boston Harbor right up to Labrador, the shore is rock, with many headlands, reefs, and islets, separated by shallow coves or by swift tidal runways. This is good hunting-ground for the seaside naturalist, and one visit to the space left uncovered at low tide will be no more than a glance at what might easily keep us busy and interested a whole summer through.
As the water ebbs away, the tops of the ledges and boulders emerge like the hairy heads of some sea-monsters, for they are mostly overgrown with long tresses of olive-brown rockweed and green ribbons of sea-cabbage, (Ulva), which trail, wet and shining, down their sides. Step carefully, for it is all extremely slippery. Do you hear that continual popping under your feet? That means that you are crushing the little bladder-like swellings strung like big beads on the stems of the rockweed. They are filled with air, and keep the long and heavy stems and leaves of the weed afloat, as you may see if you look down where it is swishing back and forth in the lapping waves. These plants must be exceedingly strong to resist the pulling and pounding of the surf in a storm; and their power to keep afloat by means of these gas-filled "bladders" is of assistance, not only in enabling them to hold together, but to form a breakwater which protects the rocks and ledges they cover from being beaten to pieces by the surf.
Underneath and upon these masses of seaweed hide a great quantity and variety of small plant and animal life, some of which we shall be able to find and study, though a large part of it requires more thorough work than we have time for, and the aid of a microscope.
But first let us look at some of the bare places, where there is no seaweed. Here is a black rock with white patches of rough little things growing upon it by the hundred. They are not mollusks, however, but rock-barnacles (see page [407]), which English boys call acorn-shells. They are small and distant cousins of the crabs.
The story of these barnacles is a very curious one. When first they hatch from the eggs which older barnacles have cast out into the sea, they are not in the least like their parents, but are queer little round-bodied creatures, smaller than pin-heads, with six feathery legs by which they paddle about, one round black eye, and two feelers. Every two or three days they throw off their skins, as caterpillars do, and appear in the new ones which have formed underneath; and every time they do this they change their shape, so that sometimes they are round, and sometimes oblong, and sometimes almost triangular!
At last they reach their full size. Then they cling with their feelers to the first rock, log, or other hard thing they come to, and pour out a drop or two of a very strong cement, which hardens around them and fastens them firmly down. After this they never move again; but a day or two later they change their skins once more, and appear as perfect acorn-shells.
Now look at one of them carefully through this magnifying-glass. Do you see that there is a little hole in the top of the shell, which is made of several pieces? That is the hole through which the animal inside fishes for food. If you were to watch it when the rocks are thinly covered with water, you would see that it kept poking out a net-like scoop, and then drawing it in again. This net really consists of the hairy legs; and as they wave to and fro in the water they collect the tiny scraps of decaying matter on which the little creature feeds. They also bear the gills by which the barnacle refreshes its blood.
You must be very careful not to knock your hand against these shells when you are hunting about among the rocks, for their edges are so sharp that they cut almost like knives.
"Another sort of barnacle," you say you have found? No: there are other sorts—the strange goose-barnacle, for instance, which attaches itself to the bottoms of ships—but what you have found is one of the limpets, and that is not a crustacean, but a gastropod mollusk. It is shaped like a tiny rough mountain, or rather like a volcano, for you see there is a hole in its summit; and we call it the keyhole limpet on account of the shape of that hole. Pick it up. Oh! you can't, eh? Of course not. Pull and push as hard as you like, you won't be able to move it, nor can the heaviest waves wash it off.
Would you like to know why?