Now look at the mouth of this spiky sea-urchin. You will find it in the very middle of the lower part of the body. Do you see what great teeth it has? There are five of them arranged in a circle as in the mouth of a starfish, and they are made in just the same way as the front teeth of a rat or a rabbit, that is, they never stop growing all through the life of the animal, so that as fast as they are worn away from above they are pushed up from below, and thus always keep just the proper length and sharpness.
Sea-urchins are rather few and small along the shores of southern New England, but more numerous northward, and on rocky bottoms offshore. On the offshore bottom there lives also a queer sort whose shells are often cast up and are well known to the children as sand-dollars.
These are about the size and shape of one of mother's cookies, and are covered with a stiff brown fur of short spines. On one side—the under one—is the little mouth, and around it the faint outlines of five radiating arms, each sketched, as it were, by a double row of "pin-pricks" where the almost invisible feet are pushed out. These sand-dollars are creeping about at the bottom in myriads where the water is a few fathoms deep; and storms cast up thousands upon the beaches or into the tide-pools, where very likely we may find some in the course of our next visit to the ocean-side.
IV
BETWEEN TIDE-MARKS
We must start early on our walk today, as soon as the tide falls away from the piece of rocky shore we have in mind, so that we may have plenty of time; for the field which we have left until the last is the richest the seaside naturalist has to explore.
As the sea sinks away it uncovers not only the weedy ledges which we studied the other day, but also spaces between them of low rocks and loose stones half sunk in mud and sand. There is much to interest the botanist, too, but he will have to look out for himself. We have more than enough to do to look after the animals.
Many dead shells are lying about, showing the various species of shell-fish which inhabit this shore or the waters of the offing. Some of them we already know, and others we can never expect to get alive except by dredging. Such are the scallops, which rarely come up as far as low-water mark, in spite of their wandering habits; and the jingleshells or goldshells, although these, like the young oysters to which they are closely related, may usually be found clinging to stones, where they seem swollen scales or "blisters" of thin amber, or gold-colored horn. There is one—let us examine it. We can't pick it off, or even pry it off; but when we slip a knife-blade slowly beneath it, it comes loose, and we discover that this queer creature is a bivalve mollusk looking (and tasting) like an oyster, and with a small flat shell underneath the bulging top one. In this undershell is a large hole, through which passes a stout stony stalk which anchors this creature as firmly as an oyster is fixed by the cementing of its undershell to whatever it has attached itself when young.
The jingleshells are extremely numerous all along the coast south of Cape Cod, wherever the water is no more than about seventy feet deep, especially in Long Island Sound; and the oystermen gather them from the beaches and from their dredgings, and scatter their shells over the floor of the sound as "seats" for young oysters. They are especially useful for this purpose because they are so slight and brittle that when, as often happens, two or three minute oyster-larvæ settle down on one of these shells, they will, as they grow, break it apart by the strain, and then each oyster, relieved from the crowding of its mates, will form a round, nicely shaped shell instead of a narrow or misshapen one, and consequently be more valuable when it comes to be dredged up, after a couple of years or so, and offered for sale.
This rough space between tide-marks is a fine place for crabs. We have seen some of these creatures already, elsewhere; and our book (see [Chapter XXXV]) has already instructed us as to the general characteristics of crustaceans. Here, scrambling about the ledges just under water, are big rock and Jonah crabs, but not so many of them as you might see in Maine. Both are eaten when "soft-shells," but are not so good as the blue crab. Here, too, are lively and pugnacious fiddlers and some green or stone crabs, wonderfully active little creatures, which in England are sent to market, but on this side of the ocean are used only for bait.
Still more comical and interesting is one of the spider-crabs, which may be called thornback. It has a little body, but very long legs, so that a big male thornback might cover eighteen inches in the stretch of its legs.