Name the four 'hallmarks' of chordates and explain the principal function of each of them.

Explanation

The hall marks of chordates : All the chordates exhibit four fundamental characters. They are Notochord, Dorsal tubular nerve cord, Pharyngeal slits or clefts and Post-anal tail.
Notochord : It is a flexible rod like structure situated along the mid dorsal line between the gut and the nerve cord. It is derived from the embryonic chorda mesoderm. It is firm but flexible. It is present throughout the life in the lancelets and cyclostomes. It is present in tail of the tadpole larva of an ascidian, It is present in embryonic stages, but is replaced partly or wholly by the vertebral column in the adults of higher chordates. Reminants of notochord occur as nuclei pulposi in the intervertebral discs of mammals.
Dorsal tubular nerve cord : A single, hollow tubular and fluid filled nerve cord is situated above the notochord and below the dorsal body wall. It is derived from ectroderm of embryo. In the higher chordates, it gets enlarged to form a distinct brain at the anterior end the rest of it becomes the spinal cord.
Pharangel slits of clefts : These are slit opening present on the Pharyngeal wall and meant for the exit of the water from the pharyngeal cavity. They are present throughout the life in the protochordates, fishes and some amphibians. These are present in larval stages in amphibians. They develop by inpushing of ectoderm and corresponding out pursing of the endoderm. In land vertebrates the gills become vestigial and nonfunctional and are restricted to embryonic stages only.
Post-anal tail : Chordates have a tail extending posterior to the anus. It losts in many species during the late embryonic development. It contains skeletal elements and muscles, coelom and visceral organs are absent in it.