Judith Peterson, age 13, of Seattle, Wash., for her question:
What are echinoderms?
The derm part of this word is coined from an older word for skin. The rest of the word comes from an older word meaning spikes or spines, The echinoderms, then, are the spiny skinned ones, On the basis of their scientific name, we would expect them to be a group of prickly animals, including perhaps the porcupine and the hedgehog. But this is not quite so. Some, but not all, of the echinoderms are prickly fellows but neither the porcupine nor the hedgehog is an echinoderm.
Though there are about 5,000 different members of the animal phylum Echinodermata, none of them are found on land, not even in or near fresh lakes and rivers. All of them are strictly sea dwellers where they enjoy life except in the cold polar oceans. Though not all the echinoderms are prickly, they all have one amazing feature in common. We and most of the animals we know are two sided. The echinoderms all are five sided.
The one we know best has five little pointed arms, Instead of spines, he is ad.orned with neat rows of stubby buttons and we are most likely to find him in tidal pools. Yes, this echinoderm is the starfish who is not really a fish at all, Some scientists feel very strongly that the starfish is mis named and they would like to re name him the sea star. In most starfish, the five sided feature is very plain to see. In some, however, the spaces between the five arms may be filled inland in others there may be 10 or even 20 arms. In any case, the starry little body, as with all the echinoderms, is based on a five sided plan.
The sea urchins are the most prickly echinoderms. It is easy to see their spines, but not so easy to see the five sided design of their bodies. However, when a sea urchin dies his prickles soon fall off and it is easy to trace the five sided design on his chalky skeleton, This fellow is closely related to the sand dollar.
The brittle stars are found in deep waters. These are green or brown and look somewhat like ordinary starfish with long, slender arms. Also like the starfish, their spines are no bigger than buttons. The feather stars also resemble the starfish, though each of their five arms branches into two. These ten arms are slender and graceful as waving plumes. The sea lilies are also echinoderms. Though each looks like a lily on a stalk, it is actually a little sea creature. The chubby sea cucumber is also an echinoderm.
The stars, urchins, dollars, feathers, lilies and cucumbers of this strange clan are divided into five classes. The starfish belong in one class. The sea urchins and sand dollars belong in another while the feather stars and sea lilies belong in still another. The brittle stars and the sea cucumbers each have a class of their own.