What are chordates?
Chordates are animals that, at some point in their life, have four features:
- A notochord, a flexible rod along the back that gives support.
- A hollow nerve tube above the notochord on the back side. (In other animals the main nerve cord is solid and runs on the belly side.)
- Gill slits in the throat region. In land animals they appear only in the embryo.
- A tail that extends past the anus.
Chordates also have the gut below the nerve tube and a heart on the belly side. Animals without these features, such as worms, insects and snails, are called non-chordates.
The lancelet
The lancelet (Amphioxus) is a sea animal about 5 cm long. It lives half-buried in sand and filters tiny food from water. Its body is pointed at both ends, like a small surgeon's knife (a lancet), which gives it its name.
It keeps all four chordate features all its life: the notochord runs from head to tail, the nerve tube is above it, many gill slits filter food and a tail extends past the gut. It has no skull and no backbone, so it is a chordate but not a vertebrate. It shows how chordates may have looked long ago.
From notochord to backbone: vertebrates
In vertebrates the notochord is present in the embryo but is replaced by a backbone (vertebral column), made of small bones called vertebrae. The front end of the nerve tube grows into a brain protected by a skull. The backbone protects the spinal cord, and gives strong support and a place for muscles to pull on.
There are five classes of vertebrates:
- Fish: gills, fins, scales, live in water.
- Amphibians: young live in water, adults on land and water.
- Reptiles: dry scaly skin, lay eggs on land.
- Birds: feathers, wings, warm-blooded.
- Mammals: hair, milk for the young, warm-blooded.
Vertebrate or not?
| Animal | Chordate? | Vertebrate? |
|---|---|---|
| Lancelet | Yes | No (notochord only) |
| Rohu fish | Yes | Yes |
| Earthworm | No | No |
| Human | Yes | Yes |
All vertebrates are chordates, but not all chordates are vertebrates.
Try it
In the 3D: go to the last step. Switch off the notochord. Then drag the Backbone slider. Which parts are still showing?
On your body: run your fingers down the middle of your back and feel the bumps of the backbone. Count how many bumps you can feel in your neck and upper back.
Key formulas and definitions
- Chordate = notochord + dorsal hollow nerve tube + gill slits + post-anal tail (at some stage)
- Vertebrate = chordate with a backbone (notochord replaced by vertebrae) and a skull
- Lancelet = chordate, not a vertebrate
- Five vertebrate classes: fish, amphibians, reptiles, birds, mammals
Worked examples
1. Is a frog a chordate? Is it a vertebrate? Give a reason.
Yes to both. A tadpole shows the notochord, nerve tube, gill slits and tail. The adult frog has a backbone.
2. Where is the main nerve cord in a chordate compared with an earthworm?
In a chordate it is a hollow tube on the back (above the gut). In an earthworm it is a solid cord on the belly side.
3. A backbone of a rabbit has 50 vertebrae. A snake has about 6 times as many. About how many does the snake have?
50 x 6 = 300 vertebrae.
Common mistakes
- Thinking all chordates have a backbone. The lancelet has only a notochord.
- Believing the notochord is the same as the backbone. The notochord is a soft rod; the backbone is made of bones.
- Saying chordates have the nerve cord on the belly. It is on the back, above the notochord.
- Forgetting that gill slits may be present only in the embryo of land animals.