George Kroner, age 12, of Brookfield, Ohio, for his question:
Is there an animal called an arthropod?
Chances are, you have seen at least a million arthropods. You have admired some of them, swatted a few and squashed a few and maybe doused a few with chemical sprays. Perhaps you have fished for arthropods and almost certainly you have enjoyed eating some of them.
"Arthropod" is not the common everyday name for a creature. It is a scientific name and like most such terms, it has clues that help us to identify the animal. "Arthropod" is coined from two older words meaning "joint" and "foot." A jointed foot does not seem to be a remarkable feature because most of the animals we know have jointed bones in their feet. But in this particular animal it is remarkable. He happens to have no bones, no internal skeleton at all. His remarkable joints are in his skin. What's more, they are not limited to his feet. He has many similar joints in his legs and often also in the antennas on his head.
No, he is not a Martian or some stranger from outer space. He is a smallish earthling and very well known to all of us. In fact, the earth's arthropods outnumber all the other animals. They include the butterfly and the dragonfly, the shrimp and the spider, the crab and lobster and perhaps a million other species. None of them are very large animals because a large body requires the, support of a bony internal skeleton. An arthropod's internal body has muscles and soft organs, nerves and blood vessels but no bones.
Such a body needs outside support to hold it in shape and protect it. And this the arthropod has. The crab has a crusty shell. Many insects have thick, leathery skins. Such coatings are stiff, too stiff to bend and an animal must be able to bend in order to move around. This is where those special joints come in. Where you would expect to find elbows and knees and other joints, the arthropod has pliable circles of softer material set into his stiff coat. These are the joints where his legs can be bent. The shell with its jointed sections is called an exoskeleton an outer skeleton.
Animals of this sort all belong in the huge phylum Arthropoda. The phylum is subdivided into a number of classes and one of these is the teeming class called Insecta. It includes, of course, all the insects. So far, more than 600,000 species of insects have been identified and scientists estimate that far more than this are still unclassi¬fied. The insects alone outnumber all other species and certainly arthropods are the most plentiful animals in the world.
The phylum Arthropoda dates back to the trilobites that thrived in the ancient seas some 500 million years ago and later became extinct. An arthropod became the first earthling to dwell on the dry land. This daring fellow was a scorpion. Fossil records show that he left the sea for a new life in the land at least 300 million years ago.