<P> A peripheral nervous system branches out from the nerve cord to innervate the various systems . The front end of the nerve tube is expanded by a thickening of the walls and expansion of the central canal of spinal cord into three primary brain vesicles: The prosencephalon (forebrain), mesencephalon (midbrain) and rhombencephalon (hindbrain), further differentiated in the various vertebrate groups . Two laterally placed eyes form around outgrowths from the midbrain, except in hagfish, though this may be a secondary loss . The forebrain is well developed and subdivided in most tetrapods, while the midbrain dominates in many fish and some salamanders . Vesicles of the forebrain are usually paired, giving rise to hemispheres like the cerebral hemispheres in mammals . </P> <P> The resulting anatomy of the central nervous system, with a single hollow nerve cord topped by a series of (often paired) vesicles, is unique to vertebrates . All invertebrates with well - developed brains, such as insects, spiders and squids, have a ventral rather than dorsal system of ganglions, with a split brain stem running on each side of the mouth or gut . </P> <P> Vertebrates originated about 525 million years ago during the Cambrian explosion, which saw the rise in organism diversity . The earliest known vertebrate is believed to be the Myllokunmingia . Another early vertebrate is Haikouichthys ercaicunensis . Unlike the other fauna that dominated the Cambrian, these groups had the basic vertebrate body plan: a notochord, rudimentary vertebrae, and a well - defined head and tail . All of these early vertebrates lacked jaws in the common sense and relied on filter feeding close to the seabed . A vertebrate group of uncertain phylogeny, small - eel - like conodonts, are known from microfossils of their paired tooth segments from the late Cambrian to the end of the Triassic . </P> <P> The first jawed vertebrates appeared in the latest Ordovician and became common in the Devonian, often known as the "Age of Fishes". The two groups of bony fishes, the actinopterygii and sarcopterygii, evolved and became common . The Devonian also saw the demise of virtually all jawless fishes, save for lampreys and hagfish, as well as the Placodermi, a group of armoured fish that dominated the entirety of that period since the late Silurian . The Devonian also saw the rise of the first labyrinthodonts, which was a transitional form between fishes and amphibians . </P>

When did the first vertebrates appear on earth