The chordates exhibit an astonishing diversity in form, physiology and habits. The number of chordate species is limited. About 49,000 species are on record which are only half of the living species of molluscs and less than one tenth of arthropods. Despite their modest number of species, the chordates make remarkable contribution to the bio-mass of the earth. Nearly all of them are medium to large in size. The vertebrates in particular are considerably larger and many of them are among the largest of living animals. The gigantic blue whale which is 35 meters long and 120 tons in weight is the biggest known animal. The smallest vertebrate , philippine goby is a fish, measuring only 10 mm in length. The chordates are able to occupy various kinds of habitats. They have adapted themselves to more modes of existence than any other group. They are found in the sea, in freshwater, in the air and on all parts of land from the poles to the equator.
General Characters :
The three distinctive characteristics of the chordates are the presence of notochord, dorsal tubular nerve cord and pharyngeal gill slits.
1. Notochord : During the embryonic development of a chordate there appears a supporting rod called the notochord. It lies dorsal to the alimentary canal and ventral to the nerve cord. In some chordates this structure persists throughout life. In others it is partially or completely replaced by a ‘backbone’. It is made up of separate bony elements or vertebrae. Structurally it is composed of large number of specialized vacuolated cells. It is surrounded by fibrous and elastic sheath. The stiffness of the notochord is due to the turgidity of fluid-filled cells and surrounding connective tissue sheath.
2. Dorsal tubular nerve cord
The nerve cord lies just above the notochord and remains entirely outside the coelom. It is a tubular structure having a small hollow canal running from one end to the other. The dorsal hollow nerve cord persits throughout the adult life of almost all chordates.
3. Gill slits or Pharyngeal clefts
These are paired lateral clefts leading from the pharynx to the exterior. They are present throughout life in fishes and a few tailed amphibians. In amphibians, like frogs and toads it is found only in the larval stages. In higher vertebrates (reptiles, birds and mammals) they are embryonic and non-functional.
4. Ventral heart
The heart is chambered. It is located ventral to the alimentary canal.
5. Closed blood vascular system
In chordates, the blood passes through a continuous system of tubes namely arteries, capillaries and veins.
6. Hepatic portal system
In chordates, the food laden blood from the digestive tract passes through the capillary net work in the liver, before reaching the heart. Thus the veins originating from the digestive tract as capillaries and ending in the liver again as capillaries constitute the hepatic portal system.
The Phylum Chordata is classified into four sub phyla:
Sub phylum 1. Hemichordata,
Sub phylum 2. Cephalochordata
Sub phylum 3. Urochordata
Sub phylum 4. Vertebrata.
First three sub phyla are collectively known as Protochordates. Since the members of these sub phyla do not have a cranium or skull they are also referred to as Acrania.
The protochoradates are considered as the fore runners of vertebrata The classification of the protochordates is based on the nature of the notochord.