The phylum Chordata includes the acraniates (the lancelet) and the vertebrates — fish, amphibians, reptiles, birds and mammals. Humans belong to this phylum too. Studying the vertebrate classes in order shows clearly how animals moved from water onto land and how their structure became more complex: the heart gains chambers, the circulations separate, breathing organs improve, warm-bloodedness appears and parental care increases. Exams often ask you to compare the classes by heart structure, breathing organs, circulation, reproduction and distinctive features.
§1. Features of the chordates
Chordates have four main features:
The notochord — a flexible internal supporting rod. In vertebrates it exists only in the embryo and is then replaced by the backbone.
The nerve tube — above the notochord, on the dorsal (back) side; in vertebrates the brain and spinal cord develop from it.
Gill slits in the wall of the pharynx — for life in aquatic forms, only in the embryo in land animals.
The heart lies on the ventral (belly) side.
In invertebrates (such as annelids and insects) it is the other way round — the nerve cord is on the ventral side.
Subphyla:
Acraniates — the lancelet: lives in sea sand, keeps its notochord all its life, has no heart (blood is pushed by the contracting ventral aorta) and no skull or brain. It shows the simplest chordate body plan.
Vertebrates — skull, brain, backbone, paired limbs (fins) and well-developed sense organs.
§2. Fish
Fish are primary aquatic vertebrates adapted to life in water.
The body is streamlined, covered with scales and mucus, which make movement through water easier.
They breathe with gills: oxygen dissolved in water passes into the capillaries of the gill filaments.
The heart has 2 chambers (one atrium + one ventricle) and carries only deoxygenated blood; there is one circulation: heart → gills (blood becomes oxygenated) → organs → heart.
Cold-blooded — body temperature depends on the water.
The lateral line senses currents, pressure and vibrations.
The swim bladder (in bony fish) changes the volume of gas inside to control depth.
Reproduction: usually external fertilisation; eggs (roe) are laid in water; some migrate long distances up rivers to spawn (salmon).
Groups: cartilaginous fish (sharks, rays — cartilage skeleton, no gill cover and no swim bladder) and bony fish (carp, catfish, pike-perch, salmon). The first vertebrates to come onto land are thought to have descended from ancient lobe-finned fish.
§3. Amphibians
Examples: marsh frog, toad, salamander, newt.
They spend part of their life in water and part on land; reproduction is tied to water.
The heart has 3 chambers (2 atria + 1 ventricle) and two circulations appear; in the ventricle oxygenated and deoxygenated blood partly mix, so organs get mixed blood.
Breathing organs: lungs (simple sacs) and thin moist skin — skin breathing is very important, so the skin must always be moist.
Cold-blooded; they become dormant in winter and drought.
Reproduction: external fertilisation, spawn laid in water; development with metamorphosis — through a tadpole.
The tadpole resembles a fish: it breathes with gills, has a 2-chambered heart and one circulation, a lateral line and a tail fin. This shows that amphibians evolved from fish-like ancestors.
The skin is dry, covered with horny scales or plates, with almost no glands — this prevents water loss; they cannot breathe through the skin.
They breathe only with lungs, which are more complex than in amphibians (internal partitions), and the rib cage helps ventilation.
The heart has 3 chambers, but the ventricle has an incomplete septum, so blood mixes less; the crocodile has a 4-chambered heart.
Cold-blooded; activity depends on the surrounding temperature.
Internal fertilisation; the egg is large, rich in yolk, covered with a tough shell and laid on land; the embryo develops inside membranes.
Reptiles are the first true land vertebrates whose reproduction does not depend on water.
Some snakes (cobra, viper species) are venomous; snake venom is used to make medicines.
§5. Birds
Birds are warm-blooded vertebrates adapted for flight.
The body is covered with feathers; the forelimbs have become wings.
The skeleton is light: bones are hollow and filled with air; the breastbone has a keel where the powerful flight muscles attach.
There are no teeth but a beak; the gut has a crop (food is stored and softened) and a gizzard (food is ground).
The heart has 4 chambers, oxygenated and deoxygenated blood do not mix; the right aortic arch is kept.
Warm-blooded — body temperature is constant and high.
Double breathing: air sacs are joined to the lungs; air passes through the lungs both when breathing in and when breathing out, so gas exchange happens twice.
Internal fertilisation; they lay eggs with chalky shells and incubate them; they care for their young.
Chicks are altricial (born blind and naked — pigeon, sparrow, swallow) or precocial (downy, eyes open, walk at once — chicken, duck, goose). Many birds make seasonal migrations.
§6. Mammals
The most highly organised vertebrates.
The body is covered with hair (fur); the skin has sweat, sebaceous and mammary glands — the young are fed on milk.
The diaphragm is a muscle separating the chest and abdominal cavities and takes part in breathing.
The heart has 4 chambers, the left aortic arch is kept; blood does not mix; warm-blooded.
Teeth are differentiated into incisors, canines and molars and sit in sockets.
The cerebral cortex is highly developed, often with folds (gyri and sulci) — complex behaviour.
Groups:
Monotremes (platypus, echidna) — in Australia; they lay eggs and feed the hatched young on milk.
Marsupials (kangaroo, koala) — the young are born tiny and undeveloped and grow in the mother’s pouch.
Placental mammals — the embryo is nourished in the uterus through the placenta: insectivores, bats, rodents, carnivores, pinnipeds, whales and dolphins, even-toed and odd-toed ungulates, primates.
Heart chambers: fish 2 → amphibians 3 → reptiles 3 (incomplete septum, crocodile 4) → birds and mammals 4.
Worked example
Which features of a frog tadpole resemble fish, and what does this prove?
The tadpole breathes with gills.
Its heart has 2 chambers and there is one circulation.
It has a lateral line and a tail fin.
Answer: the tadpole is fish-like — this shows that amphibians evolved from fish-like ancestors.
Worked example
Compare a carp, a lizard and a pigeon by heart structure and the kind of blood reaching the organs.
Carp: 2-chambered heart with deoxygenated blood; organs receive oxygenated blood from the gills.
Lizard: 3-chambered heart with an incomplete septum; organs receive mixed blood.
Pigeon: 4-chambered heart, no mixing; organs receive purely oxygenated blood.
Answer: only in the pigeon do the organs get purely oxygenated blood — that is why it is warm-blooded.
Worked example
Why is a whale a mammal and not a fish?
A whale breathes with lungs and comes to the surface for air.
Its heart has 4 chambers and it is warm-blooded.
It gives birth to live young and feeds them milk; the embryo is nourished through a placenta.
Answer: the whale is a placental mammal (whales and dolphins); its streamlined body is an adaptation to water.
Check yourself
How long does the lancelet keep its notochord?
What kind of blood passes through a fish’s heart?
Which reptile has a 4-chambered heart?
Which aortic arch do birds keep?
What is double breathing?
Which group does the kangaroo belong to?
Which vertebrate class first reproduced independently of water?
Which muscle separates the chest and abdominal cavities in mammals?
Common mistakes
Writing that birds have the left aortic arch — birds have the right one, mammals the left.
Thinking fish have two circulations — they have one.
Saying reptiles breathe through the skin — amphibians do.
Calling whales and dolphins fish — they are mammals.
Writing that all reptiles have a 3-chambered heart — the crocodile’s has 4.
Classing the lancelet as a vertebrate — it is an acraniate.