PHYLUM CHORDATA AND THEIR EVOLUTION AND PHYLOGENY:

 

Evolution And Phylogeny Of Phylum Chordata:-

• Chordates are the group of animals to which vertebrates including humans belong. Like so many other phyla of bilaterian animals,

• They originated in the ocean over 520 million years ago, before or during the Cambrian period.

• Their early evolution is of interest to neuroscientists because this was when the basic structure of the vertebrate brain and spinal cord was assembled.

• Early chordates were soft-bodied so they seldom fossilized, but zoologists reconstruct their origin using the modern chordates and their relatives plus some miraculously preserved fossils.

GENERAL CHARACTERS OF CHORDATES:-

All the chordates possess four diagnostic characters either in the embryonic or adult stage.

1. Notochord: It is a solid un-jointed, stiff but flexible rod-like structure situated on the dorsal side between the dorsal hollow nerve cord and the alimentary canal.

2. Dorsal Hollow Nerve Cord: The nerve cord of chordates is always hollow and lies dorsal to the notochord.

3. Pharyngeal Gill Slits: All the chordates have at some stage of life, a series of paired narrow openings, the gill slits on the lateral sides of the pharynx.

4. Tail: It is a post-anal part of the body.Which is reduced or absent in many adult chordates.

OTHER CHARACTERS OF CHORDATES:

• These include bilateral symmetry,

• Three germinal layers,

• Segmentation,

• Organ-system level of organisation,

• Cephalization,

• Coelom,

• Endoskeleton,

• Complete digestive tract,

• Special organs for respiration and excretion,

• Closed circulatory system,

• Separate sexes,

• Gonads with gonoducts and

• Without asexual reproduction.