What is a chordate?
Phylum Chordata is a group of animals that share four features at some stage of life. All are triploblastic, bilaterally symmetrical, have a true coelom and have a complete gut with a closed or well-developed blood system.
- Notochord: a stiff but flexible rod on the back side of the gut. It is a support for muscles. In most vertebrates it is replaced by the backbone.
- Dorsal hollow nerve cord: a single tube of nerve tissue on the back (dorsal) side. In non-chordates the nerve cord is solid and on the belly side (ventral). In vertebrates the front end swells into a brain.
- Pharyngeal slits: openings in the throat (pharynx). They are gill slits in fish; in land animals they are present only in the embryo.
- Post-anal tail: a muscular tail that extends past the anus. It is lost or reduced in many adults, such as humans.
The three groups of chordates
- Urochordata (tunicates or sea squirts): the larva swims with a tail and has all four features. The adult sticks to a rock, loses the tail, notochord and most of the nerve cord and keeps only the slits.
- Cephalochordata (lancelets): keep all four features throughout life.
- Vertebrata (fish, amphibians, reptiles, birds, mammals): the notochord is replaced by a backbone (vertebral column) of bone or cartilage, and a skull protects the brain.
The lancelet (Branchiostoma)
The lancelet is a fish-shaped, transparent sea animal about 5 cm long, pointed at both ends (that is why it is called a lancelet, like a small blade). It burrows in shallow sandy sea floors with only the front end out.
- The notochord runs from head to tail.
- The nerve cord above it is hollow, with a tiny swelling at the front but no real brain or skull.
- Many pharyngeal slits in the throat work as a food filter. Water comes in through the mouth, passes through the slits into a chamber and leaves through an opening called the atriopore.
- The post-anal tail helps swimming. V-shaped muscle blocks along the body bend it side to side.
It has no true heart, no paired fins and no jaws. It filters food with sticky mucus. It is not a vertebrate because it has no backbone, which is why it is such a good model for the ancestors of vertebrates.
Chordates vs non-chordates
| Feature | Non-chordates | Chordates |
|---|---|---|
| Notochord | Absent | Present at some stage |
| Nerve cord | Solid, belly side | Hollow, back side |
| Pharyngeal slits | Absent (usually) | Present at some stage |
| Tail beyond anus | Absent | Present at some stage |
| Heart (if any) | Back side | Belly side |
Try it
In the 3D: in the last step, switch off one feature at a time. Tell yourself if the animal is still a chordate (no, it needs all four at some stage).
At home: run your fingers down the middle of your back and feel the bumps. They are vertebrae, the bone that replaced your notochord. Where would your nerve cord be?
Key formulas and definitions
- Chordate = notochord + dorsal hollow nerve cord + pharyngeal slits + post-anal tail (at some stage)
- Subphyla: Urochordata, Cephalochordata (lancelet), Vertebrata
- Vertebrates: notochord replaced by a backbone
- Non-chordates: nerve cord solid and ventral
Worked examples
1. Why is a sea squirt called a chordate even though the adult looks like a blob?
Its larva has a tail, notochord, nerve cord and slits. A chordate needs the four features only at some stage of life.
2. List the chordate features a lancelet keeps as an adult.
All four: notochord, dorsal hollow nerve cord, pharyngeal slits and post-anal tail.
3. How does the nerve cord of an earthworm differ from that of a lancelet?
The earthworm nerve cord is solid and on the belly side. The lancelet nerve cord is hollow and on the back side.
4. What replaces the notochord in a human?
The backbone (vertebral column). Only small remains of the notochord stay inside the discs between the bones.
Common mistakes
- Thinking only animals with bones are chordates. Lancelets and tunicates have no backbone.
- Saying the nerve cord of chordates is on the belly side. It is on the back (dorsal) side.
- Thinking a chordate must have all four features as an adult. Only at some stage of life.
- Mixing up "post-anal tail" with any tail. It means the body continues behind the anus.