In some countries the sea-urchins are small—not much larger than a dime. In warmer seas the urchins grow large, even as large as a large orange. People often use these larger ones for food.
The sea-urchin walks on his spines, as the cross-star-fish does on his. But as the quills of the urchin are all around him, like a ball-cover, his walk is a roll! By the little suckers he can cling to the rocks, and he can climb up their sides.
Turn over the bare urchin shell, and you will see that while at the top it has no opening larger than a pin-prick, on the under side there is a hole where the curved rays do not come entirely together. You see the urchin must have this open place for his fish-lines, and to put food into his ever-hungry mouth.
Since the sea-urchin eats so much, he must grow! Does he?
Yes, the sea-urchin grows, and it cannot cast its shell as a crab can. It has not a soft skin as you have, yet his shell is never too tight. How can the shell expand as the urchin grows?
The shell is made up of a great many little plates, or scales. As the urchin within grows and needs more room, these little scale-plates grow larger all around.
Here is a strange thing: these wee plates are set like bricks in a dome. You know the urchin is made on the five-ray pattern bent like a flattened ball.
About the body of the urchin, within the shell, is wrapped a soft, pretty, silk-like mantle. This mantle lines the shell. It takes lime from the sea-water and builds it into more shell along the five edges of these wee plates. It also adds new plates.
So, as the urchin keeps growing all the time, its mantle is building upon the plates all the time. The house or coat, whichever you choose to call it, grows with the growth of its owner. I think your mother would be glad if she could find you a coat to grow with the growth of your body.
The shell part of the urchin is gray or greenish gray. The quills are often red, brown, pink, or purple. When a number of these urchins are fast upon a rock, they look like a bed of lovely fringed flowers.