PLATE XIII
THE COCKLE (3)
This is one of the very commonest of all the creatures of the sea-shore, and you may find its heart-shaped shells lying about on the beach in hundreds and thousands. In many places, indeed, cockle-shells are found in such wonderful numbers that they are crushed up and used for covering pathways instead of gravel.
Yet you may wander about on the shore day after day for weeks together and never see a living cockle. How is this?
Well, the reason is that cockles live buried underneath the sand. If you go down near the edge of the waves when the tide is quite low, and just stand still for a minute or two and watch, you are almost sure to see first one little jet of water, and then another, and then another, come squirting up out of the sand into the air. Now these little jets of water are thrown up by cockles which are lying buried in the wet sandy mud below. For every now and then these creatures draw down a little water into their gills, through one of their siphon tubes, and when they have sucked all the air out of it they squirt it up again through the other.
Would you like to dig one of them up and look at it? Well, just take a wooden spade and try. You will find that you cannot do it, for the cockle can dig a good deal faster than you can. The fact is that he has a very strong, fleshy organ which we call the “foot,” and with this he can burrow down into the sandy mud so quickly that by the time you have dug to a depth of six inches, he will have gone down to the depth of ten or twelve.
The cockle uses this “foot” for another purpose as well, for he can jump with it. And if you did succeed in digging him out of the ground, you would very likely see him skipping about in the most active way, almost like a sandhopper!
Upon some parts of the coast another kind of cockle is found, which has its “foot” of a bright red colour. For this reason it is generally known as the “red-nosed cockle.”
PLATE XIV
THE MUSSEL (1 and 2)
Mussels are almost, if not quite, as plentiful as cockles. If you walk down underneath a pier or a jetty when the tide is out, you will often find that the pillars which support it are covered with great clusters of these creatures; and very often the rocks which are left dry at low-water are covered with them in just the same way. They fasten themselves down by means of a bundle of very strong threads, which we call the “byssus”; and these hold so firmly, that although the waves may beat upon a bed of mussels day after day all through the year, they never succeed in tearing them away.
Near the town of Bideford in Devonshire, indeed, there is a bridge which is only kept standing by means of mussels. This bridge, which is a very long one, with twenty-four arches, runs across the Towridge River, close to the place where it joins the Taw; and the tide runs so rapidly that if mortar is used to repair the bridge it is very soon washed away. So boat-loads of mussels are brought to the bridge from time to time, and these anchor themselves down so firmly by means of their byssus threads that they actually hold the stone-work together!