The Phylum Chordate includes bilaterally symmetrical, metamerically segmented, triploblastic, enterocoelomate metazoans exhibiting hights complex organization. This phylum is supposed to be one of the most heterogenous and diversified gruop of animal kingdom. The minute sessile cephalodiscus, work-like Balanoglossus, degenerate tunicates, Amphioxus and the true vertebrates have been included in this phylum.
The vertebrates are fishes, amphibians, reptiles, birds and mammals. All these animals appear to be quite different from each other .However all of them possess certain common characters. These common characters are the presence of Notochord, dorsal nerve cord and gill-slits.
PRIMARY CHARACTERS OF CHORDATE
1. Presence of notochord or chorda dorsalis:
All Chordates porsess a solid, un segmented and flexible axial rod extending the whole length of the body. It is present mid dorsally and immediately above the alimentary canal and below the dorsal nerve cord. It porsess large vacuolated parenchymatous cells and enclosed in an inner elastic sheath and outer chordal sheath of dense fibrous connective tissue.
The notochord serves s a primitive internal skeleton. It also supports the central nervous system and the segmented muscles. In vertebrates the notochord is completely replaced by the vertebral column.
2. Presence of dorsal nerve cord:
The nervous system of chordates is the form of hollow tube present middorsally above the notochord and below the body wall. It is ectodermal in origin. From the ectoderm a medullary groove is formed which lateral develops neural tolds from its edges and forms the neural tube. Its cavity is known as Neurocoel. In higher chordates the anterior part of the neural tube develops into brain and the remaining part is called spinal cord.
3. Presence of Branchial or Pharyngeal clefts:
In the life cycle of all chordates the branchial clefts are found universally at some stage. These are formed as perforations in the laterally of pharynx and open to the exterior. Such gill-clefts are developed in every chordate. In certain animals (Amphioxus) these remain throughout life. In Aquatic chordates (fishes) the visceral clefts develop vascular Lamellae gills. These clefts are known as gill-clefts or gill-slits.
In terrestrial chordates these are seen during early development but in the adults modified into lungs. In the lows chordates (hemichordate, Cephalochordate & fishes) visceral clefts help in feeding as well as assist in respiration. In certain terrestrial forms these re modified into endocrine glands.
Origin of Chordates
There is no reliable evidence in favor of chordate origin. Origin of chordates has no definite conclusions because of lack of fossil evidences.
Recent investigations reveal the relationship among echinoderms, hemichordates and chordates. Certain phosphate is reported in all the three groups which is useful for muscle contraction Echinoderms and Hemichordates are developed from the common ancestor. So the chordates might have developed from the hemichordates.
The phylum chordata is classified into three sub-phyla. (Hemichordate has been given separate phylum status recently).
Simpler forms of chordate animals with notochord completely or incompletely formed are known as protochordates. These sub-phyla cephalochordata, urochordata (formerly Hemichordate) animals may resemble the ancestors of chordates. The vertebral column and skull are not formed in these animals.
In higher chordata animals like vertebrates animals central nervous system with brain & spinal cord, vertebral column and skull are formed along with other respective systems.