Andrew Jenkin, age 12, of Toronto, Ont., Canada for his question:
WHAT IS THE PHYLUM CHORDATA?
This topic can lead into a lot of technical terms, so let's sneak up on it gently. When you were a child, it was easy to keep track of your toys and books. Later you needed boxes and shelves to keep them in order. In school you need a desk and maybe a briefcase to cope with your possessions. Most likely your school needs a file cabinet to maintain order among report cards and such.
Ages ago, when few folks traveled far from home, it was no problem to keep track of the neighborhood plants and animals. But as people moved farther afield they met strange animals and even their familiar pets had other names. Obviously it was necessary to work out a global system for sorting, classifying and naming all living things. A major filing system was needed, acceptable to scientists and understandable to people of all languages.
The basic plan was set up in the 18th century by a Swedish naturalist called Linnaeus. We still use his system. Plants and animals are separated into two kingdoms. Each kingdom is sorted into huge groups called phyla, the plural of phylum. Each phylum is subdivided into dwindling groups called classes, orders, families, genera the plural of genus and species. You can memorize the plot from the first letter of each word in the following sentence: Kings play chess on fine grained sand.
Most of the terms used are coined from Greek or Latin, supposedly understood by all scientists. The basic unit of this workable filing system is the species. All dogs belong to the family Canidae. The scientific name for the domestic dog is Canis familiaris, which gives his genus and species. His order is Carnivora, the meat eaters; his class is Mammalia, the mammals.
His phylum is Chordata, of the kingdom Animalia. This sophisticated phylum includes all the mammals, fishes and birds, the reptiles and amphibians, plus any known or unknown backboned animal. Most chordata animals have a sturdy spine of small jointed bones, supporting ribs and a bony skeleton. The vertebra joints enclose the spinal cord, with bundles of nerves connectingall parts of the body to the brain.
Eons ago, vertebrate ancestors had a gristly, pliable spine called a notocord and vertebrate embryos still go through this stage of development. For this reason it seems only fair to include the surviving notocord animals in the phylum Chordata. These notocord chordates include lancelets and certain seaworms. However, most of the chordates have skeletons and backbones that enclose the all important spinal cord.