Kevin Powell, age 13, of South Portland
What is a sand dollar?
The pale, cookie‑shaped sand dollar we find on the beach is merely a skeleton. Its diameter is about twice that of a real silver dollar and it is made of whitish, chalky material. You may swap it for another beach trophy, but this is one dollar you cannot use for cash. The flat round skeleton is also called a sea cookie. But this is one cookie you cannot eat. Even when alive, few sea dwellers care to dine on the sand dollar.
The living sand dollar is covered with a fuzzy skin, The fuzz is made of tiny spines mixed with little tube feet, or suckers. On the upper surface is the outline of a graceful five‑petalled flower. In color, the living sand dollar is a mottled blend of grays and purples. The mouth is a hole in the center of the under surface. Near the edge of the under surface is another hole through which the little fellow gets rid of water and other wastes.
You can see these two holes on the underside of the sandollar’s skeleton. You can also see the graceful five‑petalled flower etched on the top surface, If you look closely you will see that it is formed by ladders of little horizontal slits. 'this design is important.
Along with the coat of little spines, it gives a clue to the clan of the sand dollar. It belongs to the group Echinodermata ‑ the spiny‑skinned ones. And the number five seems to be a lucky number in this animal group. It includes the starfishes, most of which have five arms, and the sea cucumbers who also have five double rows of arms
The sand dollar has tiny tube feet, or walking suckers, over most of his body. He uses them with his movable spines to get around. But a set of larger tube feet poke out from the five petal design. They are used for taking in oxygen and maybe also as holdfast.
The simple little animal enjoys life at the bottom of shallow sea water. His internal organs are between the two tonne: elates which form his skeleton He is mostly plates and spines with very little flesh. This is why few sea dwellers care to eat him.
Nimble as he is, Nature has a very important place for the little sand dollar. He is one of her garbage men and things are arranged so that the little: fellow thoroughly enjoys his work. He takes in quantities of sand through the round mouth on his underside. No, he does not eat the sand. His digestive systems sorts out fragments of alga and bits of decaying organic material, which are his food. So, simply by eating, the sand dollar helps to keep the sea floor free of garbage and other litter.