2007-10-12 / Columnists

Drawing On Science

When Is A Crab Not A Crab?
Commentary by Stephen Yaeger

By this time the reader knows that a living thing does not wear a label saying whether it is an aardvark, rose, zebra, dog, cat, bee, maple tree, or some strange being from outer space. If it produces its own food, remains in one place, and looks like a plant, it's a plant. If it has to depend on other living things for its food, moves from place to place, and looks like an animal, it's an animal.

Centuries ago, the Greek philosopher

Aristotle (384-322 B.C.) classified a living thing as either a plant (tree, shrub, herb) or an animal (grouped as to where it lived: land, water, or air). But how does one classify an alligator - living on land or in water? And why place a sparrow, bat, and bee in the same group; surely they are very different even though they fly. Such a classification system was done without any regard for the organism's physical being or anatomy, its relationship to other living things, or how it adapted to its environment.

In time scientists, with the help of new instruments such as the microscope, began to observe that there are other things to consider before any living thing can be classified as a plant or an animal. Their investigations also revealed that there are living things which, because of certain characteristics, can't be labeled as a plant or an animal. For example, during the 1670s a lens maker living in Holland by the name of Anton van Leeuwenhok built a powerful microscope. (At least during the time in question: it magnified about 270X.) He observed strange little things in pond water. Since he couldn't explain what they were, he called them his "little beasties."

Scientists soon realized that there are many organisms that just can't be classified as plants or animals. So they came up with a solution: try to construct a classification system to identify, name, and group all living things according to their characteristics and relationships to other living things.

In 1753, Carolus Linnaeus, a Swedish naturalist, constructed a classification system called Taxonomic Classification, which is essentially used to this day. This is the science of

Taxonomy. In his book he classified and named all of the then- known plants and animals found in Europe. He based his classification on physical and evolutionary relationships. For example, let's compare a sparrow, a bat, and a bee. They all have wings, but the sparrow is the only one with feathers, the bat is covered with hair, and the bee has a very sparse covering of hair. But the bee has a non-living exoskeleton whereas the sparrow and bat have living endoskeletons. The sparrow and bee lay eggs, while the bat gives birth to living young. Both the sparrow and bat have four appendages; the bee has six. Even their wings developed differently. We can go on and on until we recognize the major differences and realize that each of these creatures must be grouped separately, even though they are obviously animals. In taxonomy, organisms are placed into a series of levels or taxa from the most general characteristics working down to the most specific of characteristics. A Kingdom includes all similar organisms. There are five kingdoms: Monera, Protista, Fungi, Plant, and Animal. Each kingdom is then divided into Phyla into which organisms are placed according to more specific characteristics. This separation into taxa due to more and more specific characteristics continues on in Class, Order, Family, Genus, and Species. So the bat, bird and bee belong to the animal kingdom and the bat and bird to the same phylum, but they are in different classes. The bee belongs to a different phylum but is not in the same class as, say, a spider.

Now, about naming organisms. What's the difference among the Horseshoe crab, Helmet crab, Horsefoot crab, and King crab? Absolutely nothing! They are one and the same creature, known by different names in different geographical locations. Scientists use a common name for a given organism to avoid confusion such as this. Combining the genus and species names gives an organism its scientific name. You, the reader, are classified as belonging to the Animal Kingdom, Phylum- Chordata; Class- Mammal; Order- Primate; Family- Hominidae; Genus: Homo; Species- sapiens. And you are known, scientifically, as Homo sapiens. The Horseshoe crab is Limulus polyphemus.

Now, look at the following classification of a Horseshoe crab, a Jumping spider, and a Lady crab, which answers the question of the title of this article.

Horseshoe crab: animal, Phylum, Anthropod; Sub-phylum, Chelicerate' Class, Merostomata.

Jumping spider: animal, Phylum, Arthpod; Sub-phylum, Chelicerate; Class, Arachnid.

Lady crab: animal; Phylum-Arthropod; Sub-phylum- Crustacean; Class- Malacostraca.

Note that both the Horseshoe crab and spider belong in the same sub-phylum. The crab is placed in a different phylum. So the Horseshoe crab is not a crab at all. It is more closely related to your friendly neighborhood spider.

Questions/comments? E-mail Steve: Drawingonscience@aol.com

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