Lesson 1, Topic 1
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Animal kingdom classification

The animal kingdom

Animals are multicellular organisms whose cells have no cell walls or chloroplasts. Most animals ingest solid food and digest it internally.

Animal kingdom

(Only eight groups out of 23 are listed here). Each group is called a phylum (plural = phyla).

The animal kingdom contains many phyla. Some of them are:

Vertebrates, Arthropods, Annelids, Molluscs, Nematodes.

It is not always easy to recognise an animal. For a very long time, people thought that’s sea anemones were plants, because they tend to stay in one place and their tentacles look rather like petals . Now we know that they are animals.

One of the best way to tell if an organism is an animal is to look at its cells under the microscope. Animal cells never have cell walls.

Animals are classified into many phyla. Here are just some of these phyla:

Classes in two of these phyla:

Phylum Vertebrates

Vertebrates are animals with backbones. They are divided into 5 groups called classes:

Fish, Amphibians, Reptiles, Birds and Mammals

Details of each group are given in the table below. You only need to be able to describe visible external features, but other details can be helpful.

(all aquatic)
∘ Scales
∘ Fins
∘ Eyes and lateral line
∘ Jelly-covered eggs; usually use external fertilisation
∘ Ectothermic
∘ Gills for gas exchange
(always breed in water)
∘ Moist skin
∘ Four Limbs
∘ Eyes and ears
∘ Jelly covered eggs; external fertilisation
∘ Ectothermic
∘ Lungs/skin for gas exchange
(lay eggs on land)
∘ Dry, scaly skin
∘ Four limbs (not in snakes)
∘ Eyes and ears
∘ Soft-shelled eggs; internal fertilisation
∘ Ectothermic
∘ Lungs for gas exchange
(very few are aquatic)
∘ Feathers (scales on legs)
∘ Two wings, two legs
∘ Eyes and ears
∘ Hard-shelled eggs; internal fertilisation
∘ Endothermic
∘ Lungs for gas exchange
(very few are aquatic)
∘ Fur or hair
∘ Four limbs
∘ Eyes and ears
∘ Nipples
∘ Live young (a few lay eggs)
∘ Endothermic
∘ Lungs for gas exchange
∘ Feed young with milk from mammary glands

Classification of vertebrates

  • Vertebrates are animals with backbones (part of an internal skeleton)
  • Vertebrates are divided into five groups called classes
  • Details of each group are given in the table below
Vertebrate classBody coveringMovementReproductionSense organsOther detailsExamples
FishScalesFins (also used for balance)Usually produces jelly-covered eggs in waterEyes but no ears, lateral line along body for detecting vibrations in waterCold-blooded, gills for breathingHerring, perch, shark
AmphibiansMoist skinFour limbs, back feet often webbed to make swimming more efficientProduces jelly-covered eggs in waterEyes and earsCold-blooded, lungs and skin for breathingFrog, toad, salamander
ReptilesDry, with scalesFour legs (apart from snakes)Eggs with rubbery, waterproof shell – laid on landEyes and earsCold-blooded, lungs for breathingCrocodile, python
BirdsFeathers, scales on legsWings, two legsEggs with hard shellEyes and earsWarm-blooded, lungs for breathing, beakFlamingo, pigeon
MammalsFurFour limbsLive youngEyes, ears with pinna (external flap)Warm-blooded, lungs for breathing, females have mammary glands to produce milk to feed young, four types of teethElephant, mouse


Scales – Fins – Eyes & Lateral lines – Gills


Moist scaleless skin – Eye & Ears – 4 limbs


Dry scaly skin – Eyes & Ears – 4 legs (apart from snakes)


Beak – Feather – Scales on legs – Wings – 2 Legs


Fur – 4 Limbs

Key features of the five classes of vertebrates

Vertebrate classFishAmphibiaReptilesBirdsMammals
Examplesherring, perch, also sharksfrog, toad, newtlizard, snakerobin, pigeonmouse
Body coveringscalesmoist skindry skin, with scalesfeathers, with scales on legsfur
Movementfins (also used for balance)four limbs, back feet are often webbed to make swimming more efficientfour legs (apart from snakes)two wings and two legsfour limbs
Reproductionproduce jelly- covered eggs in waterproduce jelly-covered eggs in waterproduce eggs with a rubbery, waterproof shell; laid on landproduce eggs with a hard shell; laid on landproduce live young
Sense organseyes; no ears; lateral line along body for detecting vibrations in watereyes; earseyes; earseyes; earseyes; ears with a pinna (external flap)
Other detailscold blooded; gills for breathingcold blooded; lungs and skin for breathingcold blooded; lungs for breathingwarm blooded; lungs for breathing; beakwarm blooded; lungs for breathing; females have mammary glands to produce milk to feed young; four types of teeth


There are more arthropods than any other group of animals, so they are divided into classes:

Insects, Crustaceans, Arachnids and Myriapods

Key features of the four classes of arthropods

e.g. dragonfly, waspe.g. spider, mitee.g. crab, woodlousee.g. centipede, millipede
three pairs of legsfour pairs of legsfive or more pairs of legsten or more pairs of legs (usually one pair per segment)
body divided into head, thorax and abdomenbody divided into cephalothorax and abdomenbody divided into cephalothorax and abdomenbody not obviously divided into thorax and abdomen
one pair of antennaetwo pairs of antennaeone pair of antennae
one pair of compound eyes several pairs of simple eyesone pair of compound eyessimple eyes
usually have two pairs of wingschelicerae for biting and poisoning prey exoskeleton often calcified to form a carapace (hard)

Special features of Arthropods:

  • Invertebrates (no backbone)
  • Waterproof Exoskeleton ⇢ Exist in very dry places, not confined to water or moist places like most invertebrates.
  • Segmented body
  • Jointed legs (exoskeleton prevents movement)

Four classes of Arthropods


Insects are a very successful group, due to their exoskeleton and tracheae, which are very good at stopping water from evaporating from insects’s body, so they can live in very dry places.

Key features of Insects

  • 3 pairs of jointed legs
  • 1 or 2 pairs of wings
  • 1 pair of antennae
  • 3 body parts: Head, Thorax, Abdomen
  • 1 pair of compound eyes
  • Breath through tracheae


These are the crabs, lobsters and woodlice. They breath through gills, so most of them live in wet places and many are aquatic.


These are spiders, ticks and scorpions. They are land-dwelling organisms.

Key features of Arachnids

  • 4 pairs of legs
  • no wings
  • 2 pairs of antennae
  • 2 body parts: Cephalothorax, Abdomen
  • several pairs of simple eyes
  • all have piercing jaws since all are predator
  • chelicerae (pointed mouthparts) for biting and poisoning prey


These are the centipedes and millipedes

Key features of Myriapods

  • long, thin body with many segments for moving easily through soil and leaf litter
  • no obvious thorax and abdomen
  • each segment has jointed legs (>9 pairs)
  • 1 pair of antennae as sense organs in dark habitats
  • simple eyes

7 Other groups of invertebrates

Details about some more phyla of invertebrates:

Annelids, Nematodes, Molluscus.

Phylum Annelids

Annelids are worms, with bodies made up of ring- like segments. Most of them live in water, some like the earthworm live in moist soil.

Key features of Annelids

  • many segments on long body
  • body covered with mucus to conserve water
  • mouth and anus present
  • bristles (stiff hair) usually present for movement
  • many are hermaphrodite (intersex)

Phylum Nematodes

Nematodes are worms, but unlike annelids their bodies are not divided into segments. They are usually white, long and thin. They live in many different habitats. Many nematodes live in the soil.

Key features of Nematodes

  • no segments
  • long cylindrical body
  • body pointed at both ends

Phylum Molluscs

Molluscs are soft-bodies animals, sometimes with a shell (snails) or without (slugs).

Key features of Molluscs

  • soft, unsegmented body
  • muscular foot for movement or burrowing
  • most have a shelf made of calcium carbonate (protection from predators/drying out)
  • often have eyes on retractable tentacles

Common misconceptions

Students are often confused by the different numbers of legs in insects, arachnids and curstaceans.

They often loose the mark by stating that insects have 3 legs instead of 3 pairs of legs.

Types of invertebrates:

  • Arthropods: have jointed legs, a hard exoskeleton (carapace), body divided into segments, there are different types:
    • Insects: 6 legs, 3 body parts (head, thorax and abdomen), made of many segments, and two antennae e.g. bees.
    • Crustaceans: many legs, 4 antennae, 2 body parts (head-thorax and abdomen), made of many segments e.g. crabs.
    • Arachnids: 8 legs, no antennae, 2 body parts (head-thorax and abdomen) e.g.spiders.
    • Myriapods: many legs, many segments, 2 antennae e.g. centipede.
  • Annelids: ringed worms, no legs, chaetae (bristles) e.g. earthworms.
  • Nematodes: un-segmented worms, no legs, no chaetae e.g. nematodes.
  • Molluscs: un-segmented, have gills and one muscular foot e.g. snails.

Leaf types in flowering plants

Summary of the key features of monocots and dicots

leaf shapelong and narrowbroad
leaf veinsparallel branching
grouping of flower parts
(petals, sepals and carpels)

In addition to knowing the features used to place animals and plants into the appropriate kingdoms, you also need to know the main features of the following kingdoms: Fungus, Prokaryote and Protoctist.