Voir la source de Chordates 2BIbDF09 10/11

=What are Chordates ?= 

More than 90% of all known chordates species have backbones (and thus are vertebrates). However, the phylum Chordata also includes three groups of invertebrates: lancelets, tunicates, and hagfishes
==What are the main characteristics of Chordates ?==


Chordata is the last phylum of the animal kingdom.There are four main characteristics to identify the chordates: 
1. a dorsal, hollow nerve cord
2. notochord 
3. pharyingeal slits 
4. post-anal tail 				 

==What are the different kind of Chordates ?==
===Invertebrates Chordates===
===Vertebrates Chordates===
 
:The first vertebrates appeared about 470 million years ago. They were jawless marine fish.

Although fewer than invertebrates, the group of vertebrates form a homogeneous set of animals with anatomical characteristics in common.
They owe their name to the existence of a '''skeleton made of bone''' (or '''cartilage''' in some fishes) whose essential part is the spine.
All have a nervous system consisting of an anterior part, the '''brain''', lodged in the skull, and a posterior, the '''spinal cord''', housed in a cavity (the spinal canal) formed by all the vertebrae.
They also have a '''circulatory system''' formed by the '''heart''' and blood vessels ('''arteries and veins'''), in which circulates the blood that transports oxygen for '''breathing'''.

Vertebrates are the '''most advanced''' animals and closest to the man (who is also a vertebrate). Moreover, they are almost all the useful species to man. They live in '''all environments''', in water or on land.
The first vertebrates were '''fishes''', which some species have become '''amphibians'''. The '''reptiles''' were formed from these amphibians. '''Birds''' and '''mammals''' come from reptiles.[[Utilisateur:NathalieR|NathalieR]] 9 janvier 2011 à 18:21 (CET)

====Classification of the vertebrates====
:Vertebrates are divided into '''5 main groups''', or classes, each characterized by a particular lifestyle and a special anatomy.

*First appeared the '''fishes''', during the Paleozoic era, '''470 million years ago'''. They are aquatic animals that breathe through '''gills'''; they have no legs but '''fins''', and their bodies are usually covered with '''scales'''. These are the most numerous and most diverse of the vertebrates: there are about '''30,000 species'''.
*'''Amphibians''' (frogs, newts, salamanders) were formed from fishes that, '''360 million years ago''', began to leave the water and '''move onto land'''. These are the first vertebrates to have '''4 legs'''; the young have '''gills''', adults, '''lungs'''. Unlike fish, their skin is '''naked''', that is to say, without scales. There are only '''3000 species''' that are mainly found in '''wetlands'''.

*The '''reptiles''' were born at the end of the Paleozoic era, '''290 million years ago'''. Most have '''4 legs''', but arranged in such a way that they '''badly support the body'''. That is why they move by '''crawling'''. This mode of travel is even more pronounced in snakes, which are legless reptiles. The reptiles were numerous and varied in the '''Mesozoic era''', the so-called because of this "'''Age of Reptiles'''"(or "age of dinosaurs", which are typical reptiles at the time). It remains today only a '''small number''' of them, including turtles, crocodiles, lizards and snakes.

*'''Birds''' appeared '''150 million years ago'''. The first "bird" known, the Archaeopteryx, is actually a kind of '''feathered dinosaur''': he possessed both reptilian characters (teeth and a long tail) and characters of bird ('''feathers'''). Nowadays, there are about '''8000 species''' of birds that inhabit all environments, including those coldest as the Antarctic continent.

*'''Mammals''' appeared roughly at the same time as dinosaurs, during the Mesozoic era, '''200 million years ago''', from very odd reptiles called "'''mammalian reptiles'''". However, they have only become numerous and varied at the '''Cenozoic era''', about 65 million years ago, when most of the reptiles had disappeared. It is for this reason that the Cenozoic era has been called "'''age of mammals'''".Mammals are identified by their bodies covered with '''hair''' and the existence of '''breasts''' producing '''milk''' used to feed the young. Unlike other vertebrates that lay eggs (called '''oviparous'''), mammals (except the platypus) are '''viviparous''': they give birth to '''fully formed''' young.[[Utilisateur:NathalieR|NathalieR]] 9 janvier 2011 à 19:44 (CET)