What is Chordata?
In the process of classification, the Animal Kingdom is divided into several major animal groups called phyla (singular, phylum). There are altogether 30 animal phyla currently identified. The last major group of the Animal Kingdom is known as the phylum Chordata. It was created in 1880 by Balfour. The name of this phylum is derived from two Greek words, the chorde meaning a string or cord, and ata meaning bearing. The reference is to a common characteristics feature in the form of a stiff, supporting rod-like structure along the back, the notochord (Gr., not on, back; L., chorda, cord), which is found in all the member of the phylum at some stage of their lives. Thus, chordates are animals having a cord, I.e., notochord. The animals belonging to all other phyla of the animal kingdom are often termed 'the non-chordates' or 'the invertebrates' since they have no notochord or backbone in their body structure.
Chordate Animals
Phylum Chordata is the largest of the deuterostome phyla. It is the highest and the most important phylum comprising a vast variety of living and extinct animals including the man himself. Most of the living chordates are well known familiar vertebrate animals such as fish, amphibians, reptiles, birds, and mammals. Besides, they include several marine forms such as the tunicates and lancelets, that are less well known.
Diversity of Chordates
The chordates exhibit an astonishing diversity of form, physiology and habit.
Numerical strength. The number of chordate species is not unusually large. About 49,000 species are on record which are only half of the living species of molluscs, and less than one-tenth those of arthropods. The two sub phyla Urochordata and Cephalochordata claim for nearly 2,500 species. The subphylum Vertebrata includes 46,500 species; of these, fishes are the most numerous with approximately 25,000 species. It is commonly assumed that amphibian species number about 2,500, reptiles 6,000, birds 9,000 and mammals 4,500.
Size. Despite their modest number of species, the chordates make a disproportionate contribution to the biomass of the earth. Nearly all of them are medium to large in size. The vertebrates, in particular, are considerably large and many of them are among the largest of living beings. The gigantic blue whale (Balenoptera musculus), which grows to 35 meters long and 120 tons in weight, is the biggest known animal. The whale shark (Rhineodon typus) reaches a length of 15 meters and is the second biggest vertebrate after whales. The smallest fish is a Philippine goby (Pandaka) measuring only 10 mm long.
Ecology. The chordates are not only the largest animals in existence today, but ecologically they are among the most successful in the Animal Kingdom. They are able to occupy most kinds of habitats and they have adapted themselves to more modes of existence than any other group, including the arthropods. They are found in the sea, in freshwater, in the air, and on all parts of land from the poles to the equator. Birds and mammals have been able to penetrate cold climate because they have a constant body temperature, something no other animals have.
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