PHYLUM – CHORDATA
(Gr. Cord = string; ata bearing)
Introduction: The organisms which bear a stiff supporting rod-like structure along the back called a notochord at some stage of their lives. Chordates have three important common features, namely the presence of a notochord, a dorsal tubular nerve cord and pharyngeal gill clefts. The phylum contains about 65,000 existing (living) and 25,000 extinct species. Existing species inhabit land, water or air throughout the world. Fish, frogs, lizards, snakes, birds, rats, domestic animals, man, etc., are a few examples of common chordates. Generally, chordates are relatively larger and better organised than other metazoans (i.e., nonchordates). The representatives of this phylum exhibit great diversity in their size, habits, habitats, anatomy, physiology, etc. These animals probably originated prior to the Cambrian period, but there is considerable controversy regarding their origin.
General Characters:
1. Presence of notochord: It is a rod-like supporting structure and located below the nerve cord and above the alimentary canal. It may either persist throughout life (e.g., in lampreys) or it may be replaced partially or completely by the vertebral column.
2. Presence of dorsal tubular nerve cord: It is a hollow tube filled with fluid and dorsal in position. It lies above the notochord and outside the coelom. It persists throughout life in most of the chordates, but in a few it degenerates before maturity.
3. Presence of pharyngeal gill clefts: Gills are the respiratory structures. Gill clefts are the paired openings leading from pharynx to the exterior. The gills are found throughout the life in fishes, but in amphibians the gills appear only in larval stages; later the gills become degenerate during the adult stages.
4. They are aquatic, aerial or terrestrial. All are free-living with no fully parasitic forms. 5. Bilateral symmetry: In chordates, the formation of two equal body parts when we cut the body in only one plane from the central axis.
6. Metamerically segmented body: The body is metamerically segmented. Externally, the segmentation is less distinct.
7. Ventral heart: The heart is ventral in position.
8. Closed type of circulatory system: RBCs are suspended in the plasma. Haemoglobin is the respiratory pigment in the RBCs.
9. Triploblastic animals: The body wall is made up of three germinal layers, namely outer ectoderm, middle mesoderm and an inner endoderm.
10. Hepatic portal system: The food-laden blood from the alimentary canal is carried to the liver through a hepatic portal vein.
11. Paired appendages: In all vertebrates, pectoral and pelvic girdles are present. These are paired in number, and in some animals it may be modified. Ex. fishes, frogs, reptiles, birds and mammals.
12. Post-anal tail: A post-anal tail usually projects beyond the anus at some stage and may or may not persist in the adult.
13. Endoskeletons may be cartilage or bony.
14. Digestive system complete with digestive glands.
15. excretory system includes protonephric, mesonephric or metanephric kidneys.
16. Exoskeleton well developed in most of the vertebrates.
17. Nervous system consists of a dorsal and tubular nerve cord which, at the anterior end usually enlarged to form a brain.
18. Sexes are separate and development is direct with few exceptions.
Unique features of Chordates
1. All chordates possess a notochord. It is an elastic, solid skeletal supporting rod. It runs along the long axis of the body below the nerve cord and above the alimentary canal. It persists throughout life in some primitive chordates. In higher forms it is replaced by a vertebral column (backbone).
2. A dorsal tubular nerve cord is present in all chordates. It is placed dorsal to both the alimentary canal and notochord. The anterior part of the nerve cord is modified into a brain.
3. A series of paired Pharyngeal gill clefts are present on the side walls of the pharynx. The gill slits persist throughout the life in fishes. In amphibians, they are present only in the larval stages. In reptiles, birds and mammals, the gill slits make their appearance only in the embryonic stages and are non-functional.
4. The endoskeleton, a post-anal tail and a ventral heart are the other unique characters.
5. They are bilaterally symmetrical, triploblastic, eucoelomate with organ system levels of organisation.
6. The body is differentiated into head, trunk and tail.
7. Organ systems are well developed.
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