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Karen Swain, age 11, of Indianapolis, Ind., for her question:

WHAT ARE CHORDATES?

A fox is a chordate animal, so is a frog. Tunas and turtles, sharks and sardines also are chordates. Altogether, more than half a million different animals are classed as chordates. And the group also includes people.

Biologists often use the term chordates when they mean the vertebrates, or backboned animals. These are the creatures that have jointed, bendable spines made of bone or gristle. A vertebrate also has a head and a skull to encase some sort of brain. He has a right and a left side. Usually he has four limbs, though some limbs may be fins, flippers or wings.

In classifying the animal kingdom, zoologists often disagree on minor details. But they all agree on the large groups called phyla, the plural of phylum. The most advanced of these is the phylum Chordata, meaning string or cord. This feature is the string or notochord that supports the nerve cord  which becomes the central spinal column. This, of course, brings us to the backbone feature, the spine of the vertebrates.

The big phylum is divided into smaller classes of more closely related animals. For example, some 9,000 chordates belong in Aves, the bird class, and about 6,000 others belong in the reptile class. Another 3,000 or so belong in the amphibian class and about 5,000 are mammals of the class Mammalia.

The largest group of chordate vertebrates are fishes, which are sorted into two groups. The sharks and their relatives are classed as cartilage fishes because their skeletons are made of gristly cartilage. The rest are classed as the bony fishes.

Several groups of more simplified animals also belong in the phylum Chordata. One is a group of worms with a very short notochord. Another has the notochord only in the larval stage. Another includes a fishy group of simple creatures called lancelets.

In some cases, a chordate feature is present only during the larval or embryo stage of development.  For example, chordates are said to have gills  though in many species the gills exist only for a .time during the embryo stage. By the time the animal is born or hatched, the gills have become air breathing lungs.

 

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