Perhaps you might think that these come there by accident. But they do not. The crab himself plants them there! If you keep him in an aquarium you may often see him doing so. First of all he turns one of his long claws over his back and scratches away at the carapace, so as to roughen the surface. Then he pulls up a little sprig of sea-weed or sponge and actually plants it on his shell, pressing the rootlets firmly down. And besides the spikes upon the shell there are numbers of tiny hooks, which help to hold it in position. Then the crab plants another piece of weed or sponge in just the same way, and so he goes on planting piece after piece until his back is completely covered.
Now why do you think he takes all this trouble?
Well, the reason is that he does not want to be seen; for he has a great many enemies, and he knows perfectly well that if he were to lie among the sea-weeds or sponges at the bottom of the sea they would be quite sure to notice him as they passed by, and then he would almost certainly be killed and eaten. So he clothes himself with either sea-weeds or sponges, as the case may be, and then feels that he is perfectly safe, and that as long as he keeps quite still even the sharpest eye will fail to notice him. And if you catch one of these crabs which is covered with sea-weeds and put it into an aquarium in which sponges are growing, it will very soon strip the weeds off its back and cover itself with sponges instead; while if you catch one that is covered with sponges, and put it into a tank in which sea-weeds are growing, it will strip off the sponges and cover itself with sea-weeds!
1. THE MASKED CRAB.2. THE THORNBACK CRAB.
The thornback crab often grows to a rather large size. Indeed, next to the edible crab, it is the largest of all the crabs which are found in our British seas, for its carapace is sometimes as much as eight inches long and six inches wide, while its great claws may be fourteen or fifteen inches in length. On some parts of the coast it is used for food, but its flesh is rather coarse and of poor quality.