Concept preview

What you will be able to do

1. 01List and describe the four features shared by all chordates.
2. 02Explain how vertebrates differ from the other chordates.
3. 03Describe the five vertebrate classes and a key adaptation of each.
4. 04Distinguish ectotherms from endotherms and explain the trade-offs between them.

The chordates (phylum Chordata) are the animal group to which we ourselves belong, and they are defined not by having a backbone — many chordates do not — but by four distinctive features that appear at some stage of the life cycle, even if only in the embryo. The phylum includes the familiar vertebrates (fish, amphibians, reptiles, birds and mammals) but also two groups of small, backbone-less marine animals: the tunicates (sea squirts) and the lancelets. These humble invertebrate chordates matter enormously, because they show the basic chordate body plan stripped of the complications that vertebrates later added, and they reveal that the features we think of as uniquely 'ours' are in fact very ancient and shared across a much wider group of animals.