But, you say, you have heard of "hard-shell" and "soft-shell" crabs, and want to know the difference? It is simply a difference of condition. If you will turn to page [397], you will find described that extraordinary process by which crabs grow, by throwing off their stiff old skins and expanding to fill the elastic new one which has formed underneath. Before this change, the creature is a "hard-shell" in fishermen's language, and just afterward, when he is large and tender, he is naturally a "soft-shell"; and then is the time to eat him.
Notice how the black masses of peat along the banks are honeycombed with holes, as if somebody had been pushing down the point of his umbrella. They are the homes of little fiddler-crabs, which scuttle into them by the hundred as we approach, and then creep up to peer out after we have passed by, and make sure it is safe to go abroad again. In other holes live two other sorts of burrowing crabs. One is the little mud-crab (Panopæus), which is a peaceful cousin of the fiddler; and the other is the sand-crab (Ocypoda) whose peculiarity it is to be perfectly sand-colored, so that it is almost impossible to see him until he moves; consequently he is commonly found only in the sandy places.
As we float nearer to the mouth of our winding creek, we begin to notice bunches of mussel-shells, clinging closer to each other than grapes in a bunch; and when we try to pick one up we find it quite immovable. In fact, they are anchored to the roots of the grasses, and to each other, by a bunch of byssus threads from each mussel, like those of the pinna; and these threads are so strong that they can hold the mussels firm against the beating of the waves, so that a shore which is thickly covered with mussels is safe from wearing away. You may see an example of this in the tideway at the mouth of this very creek, and masses of mussels strengthen the supports of that wharf we are approaching. If you were to go near the town of Bideford, England, you would see a bridge of twenty-four arches, which runs across the Torridge River close to the place where it joins the Taw. Now that bridge is held together by clusters of mussels! The force of the stream is so great, that if mortar is used to repair the bridge it is very soon washed away. So from time to time large boat-loads of mussels are taken to the spot and shot into the water, and they fasten themselves so firmly to the bridge by means of their byssus threads, that they actually hold together the stones of which it is built!
These binding mussels are mostly of the smooth, dark-blue sort which are found on both sides of the Atlantic, and in Europe are gathered and eaten. When our people become a little wiser and more economical, we also will take advantage of this great stock of excellent food right at our doors.
But in the bunches which are scratching the side of the boat as we glide along close to the bank are some which are much larger, though smooth, like the edible mussels. They are an American species. Then here and there in peat you may see a sort whose shell is rough, with ridges spreading out toward the large end, and these you may call horse-mussels.
Now we have got down to the boat-landing toward which we have been lazily drifting, and we will twist the chain around one of the piles that support it, and stop long enough to take a look at one of them. Most of the time each pile is under water, and therefore is overgrown with a thick "fur" of plants and animals.
You will see that most of this fur consists of seaweeds, but their leaves are often the resting-place of several sorts of lowly animals. Indeed, you must look sharp to make sure whether some of the feathery tufts that droop from a dank old post, or spray out so beautifully in the ripples at its foot, are plants or animals. We will not talk about that just now, but wait till we take our excursion to the rocky shore, where we shall find barnacles and corallines and sea-mats and polyps bigger and better than here.
But do you see between those green fronds that roundish yellow object about as big as a filbert? Touch it gently. Did you see tiny jets of water squirt out of two little nozzles on its surface? That gives it the name of sea-squirt. Into one of the nozzles, when the tide comes over it, is constantly sucked a current of sea-water which passes into a stomach-like cavity, where the minute particles of food in the water are caught and digested; then the water passes on through another cavity where the blood receives its oxygen, as in our lungs or a crab's or fish's gills, and then rushes out. So this little object is a real animal, with heart, blood, stomach, and something in the way of nerves—enough, at any rate, to feel your touch, shrink, and squirt out all the water in its bag-like body.
There are a good many kinds and forms of these ascidians, as naturalists call them, some larger, some waving about on the summit of stalks like lily-buds, and some clustered into colonies grown together, which form bands around the stems of plants, or make masses called "sea-pork" by the fishermen, or float in chains, by millions, on the surface of the open sea.
Here, too, are small red and yellow sponges; some coarse little sea-anemones, etc.; and wandering over the whole, feeding upon one or another of these, and cleaning the polyps and polyzoans off the algæ, are a sort of marine daddy-long-legs, called no-body crabs, because they seem all legs and look crab-like.