What are chordates? How many kinds of chordates do you know? Chordates are active animals with bilaterally symmetric bodies that are longitudinally differentiated into head, trunk and tail. The most distinctive morphological features of chordates are the notochord, nerve cord, and visceral clefts and arches. There are 5 kinds of chordates in the world: fishes, amphibians, reptiles, birds, and mammals. In the essay I will talk about the common and distinguishing characteristic between each kind of chordates. .
First, the animals living in water that we know they are fishes. Fishes are poikilothermal, because they could charge their blood temperature with the temperature of the surroundings. Fishes were the earliest vertebrates and presumably evolved from a group of aquatic lower chordates; the terrestrial vertebrates evolved from fishes. Fishes can not live on the land, because they can not breathe on land. For example, if you take a fish out the water, you will discover that fish die in later. The reason is fishes breathe through gills only under the water. Fishes lay eggs for birth under the water, they have scales cover their bodies, and they can see everything behind them through their eyes. And Fishes have the backbone on their back.
Second, We know amphibians are the animals able to live both on land and in water, they are the most primitive of the terrestrial vertebrates, are intermediate in evolutionary position between the fish and the reptile. For example, typically they undergo a metamorphosis from an aquatic tadpole which only live under the water, it breathes through gills and use its tail to swim, into a terrestrial or partly terrestrial, air-breathing, four-legged adult. Amphibians lay eggs in water or in a protected place where their moisture will be conserved. They have the backbone on their back, too.
Third, reptiles are the poikilothermal animals that we know.