What makes a chordate

Four features of a chordate

Phylum Chordata holds the fishes, frogs, snakes, birds and mammals, and a few small sea animals that look nothing like them. What do they share? Four features, all seen in the plan below.

1. A notochord. This is a firm, flexible rod of cells along the back. It forms from the mesoderm of the embryo and supports the body.
2. A dorsal hollow nerve cord. A single nerve cord runs along the back, above the notochord. It is a tube, hollow inside.
3. Paired pharyngeal gill slits. The wall of the pharynx, just behind the mouth, has openings on both sides.
4. A post-anal tail. The body goes on behind the anus as a tail.

Every chordate shows all four at some stage of its life. Some keep them only in the embryo. You had gill slits and a tail before you were born, though you have neither now.

Chordates also share the features of the advanced animals: bilateral symmetry, three germ layers, a true coelom and the organ-system level of organisation. Their circulatory system is closed, so the blood stays inside vessels.

Chordates and non-chordates

Set a fish beside an earthworm or a cockroach, and five differences stand out.

Notice the flip in position. In a chordate the nerve cord is on the back and the heart is below. In an earthworm the nerve cord runs along the belly and the main pumping vessel is above.

Three subphyla

The notochord decides the subphyla. Where is it, and does it last?

• Urochordata, or Tunicata. The notochord is found only in the tail of the larva. The tadpole-like larva swims for a while, then settles. As it grows into the adult, the tail and the notochord are lost. Examples: Ascidia, the sea squirt, Salpa and Doliolum.
• Cephalochordata. The notochord runs from the head to the tail and stays for the whole of life. Example: Branchiostoma, also called Amphioxus or the lancelet. It is a small, fish-shaped animal that burrows in sand.
• Vertebrata. The notochord is present only in the embryo. In the adult, it is replaced by a vertebral column of cartilage or bone.

Urochordates and cephalochordates are together called protochordates. All of them live in the sea.

So all vertebrates are chordates, but not all chordates are vertebrates. A sea squirt is a chordate with no backbone at all.

What vertebrates add

Besides the four chordate features, vertebrates have:

• a ventral muscular heart with two, three or four chambers;
• kidneys, for excretion and for keeping the water balance (osmoregulation);
• paired appendages, which are fins or limbs.

Jaws or no jaws

Vertebrates fall into two divisions.

• Agnatha, with no jaws. Its living class is Cyclostomata, the lampreys and hagfishes.
• Gnathostomata, with jaws. It has two superclasses:
  • Pisces, the fishes with fins: classes Chondrichthyes and Osteichthyes;
  • Tetrapoda, usually with two pairs of limbs: classes Amphibia, Reptilia, Aves and Mammalia.

That gives seven classes of living vertebrates. The next two lessons meet them one by one.

Where the notochord goes

Move the slider through the three subphyla. Watch the notochord in the young stage and in the adult.

Try it in Python

The program checks five animals for the four chordate features and names each one a chordate or not.

Practise with new numbers

Common mistakes

• "Chordate means an animal with a backbone." A backbone is the mark of vertebrates only. Sea squirts and lancelets are chordates without one.
• "The notochord is the spinal cord." The notochord is a supporting rod. The nerve cord lies above it.
• "Gill slits mean the animal breathes with gills all its life." In land vertebrates the slits appear only in the embryo.
• "Every chordate keeps its notochord." Urochordates lose it as adults, and vertebrates replace it with a vertebral column.
• Mixing up the positions. Chordates have a dorsal nerve cord and a ventral heart. Non-chordates have the reverse.

Remember these

• The four chordate features: notochord, dorsal hollow nerve cord, paired pharyngeal gill slits and a post-anal tail.
• Urochordata: notochord only in the larval tail (Ascidia, Salpa, Doliolum).
• Cephalochordata: notochord from head to tail, for life (Branchiostoma).
• Vertebrata: notochord in the embryo, replaced by a vertebral column in the adult.
• Protochordates are the urochordates and cephalochordates, all marine.
• Vertebrates add a ventral muscular heart, kidneys and paired fins or limbs.
• Agnatha have no jaws (Cyclostomata). Gnathostomata have jaws (Pisces and Tetrapoda).

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Name the four features that every chordate shows at some stage of its life.

2. A cockroach has a nerve cord too. How is it different from yours?

3. Why is a sea squirt called a chordate, though the adult has no notochord?

4. Which two subphyla make up the protochordates, and where do they live?

5. State three features that vertebrates have besides the four chordate features.