Classification of Kingdom Animalia

Lessons

Senior 2 · Biology

Classification of Kingdom Animalia

5 lessons

Lesson 1 of 5

What Makes an Animal? Features Used to Classify Animals

18 min

Let's begin

Walk from the edge of Nyungwe forest down to the shore of Lake Kivu and you could meet a chimpanzee, a chameleon, a millipede, an earthworm, a snail and a shoal of isambaza. Biologists have named more than a million animal species, and many more are still undiscovered.

No one can learn a million animals one by one. Instead, biologists sort them into groups using features they can observe. Once you know the features of a group, you can say a great deal about an animal you have never seen before. In S1 you met the five kingdoms. This unit looks inside one of them, Kingdom Animalia.

What makes an animal an animal?

Animals are very different from one another, but every animal shares these features:

Eukaryotic and multicellular. Their cells have a nucleus, and the body is made of many cells.
No cell walls. Unlike plant and fungal cells, animal cells are surrounded only by a cell membrane.
Heterotrophic. A heterotroph cannot make its own food, so it takes in (ingests) other living or dead organisms. Animals may be herbivores (plant-eaters), carnivores (meat-eaters), omnivores (both) or parasites (living on or in a host).
Movement. Most animals can move from place to place (they are motile) during at least one stage of life.
Tissues. Almost all animals have cells grouped into tissues, such as nervous tissue and muscle tissue, so they can respond quickly to their surroundings.
Sexual reproduction. Almost all animals can reproduce sexually, and many can also reproduce asexually.

Worked example: is it an animal?

Question: Use the features above to decide whether (a) a mushroom and (b) a bath sponge are animals.

(a) Mushroom. It is multicellular and cannot make its own food. However, its cells have walls made of chitin, and it digests food outside its body and then absorbs it. It never ingests food. Not an animal: it belongs to Kingdom Fungi.

(b) Bath sponge. It is multicellular, its cells have no walls, and it takes in tiny food particles filtered from sea water. The adult stays fixed to a rock, but the young sponge (the larva) swims. An animal, even though the adult does not move.

The classification ladder

Biologists place every organism in a series of groups called ranks. Each rank holds fewer, more similar organisms than the rank above it. Here is the mountain gorilla of the Volcanoes National Park:

Rank	Group	What its members share
Kingdom	Animalia	multicellular heterotrophs without cell walls
Phylum	Chordata	a supporting rod and a nerve cord along the back
Class	Mammalia	hair and milk glands
Order	Primates	grasping hands, forward-facing eyes
Family	Hominidae	the great apes
Genus	Gorilla	all gorillas
Species	Gorilla beringei	eastern gorillas; the mountain gorilla is one of its two subspecies

Features used to divide the animal kingdom

Backbone. Vertebrates have a backbone (vertebral column); invertebrates do not. About 95% of animal species are invertebrates.
Body symmetry. An asymmetrical animal has no mirror-image halves (a sponge). A radially symmetrical animal can be cut into matching halves along many planes through its centre, like a cake (a jellyfish). A bilaterally symmetrical animal has only one plane that gives matching left and right halves (a goat, a grasshopper, you). Bilateral animals have a head end (anterior), a tail end (posterior), a back (dorsal side) and an underside (ventral side).
Number of body layers. In the embryo, animals form either two cell layers (diploblastic, such as jellyfish) or three (triploblastic, such as worms, insects and vertebrates).
Body cavity. A coelom (say "SEE-lom") is a fluid-filled space between the gut and the body wall that holds the organs. Flatworms have none, roundworms have a "false" cavity, and earthworms, insects, snails and vertebrates have a true coelom.
Segmentation, skeleton and limbs. Is the body divided into repeated segments? Is the skeleton inside (endoskeleton) or outside (exoskeleton)? Are the legs jointed?

The phyla in this unit

Phylum	Key feature	Example
Porifera	body full of pores, no true tissues	bath sponge
Cnidaria	radial symmetry, stinging tentacles	Hydra, jellyfish
Platyhelminthes	flat, unsegmented worms	tapeworm, planarian
Nematoda	round, unsegmented worms	hookworm, Ascaris
Annelida	segmented worms	earthworm, leech
Mollusca	soft body, muscular foot, often a shell	snail, octopus
Echinodermata	spiny skin, five-part symmetry	starfish
Arthropoda	jointed legs, exoskeleton	bee, crab, spider
Chordata	notochord and dorsal nerve cord	fish, frog, crocodile, crane, gorilla

Tip

Memory hook for the ranks: "Keza's Puppy Chased Our Fat Grey Sheep" gives Kingdom, Phylum, Class, Order, Family, Genus, Species.

Exam technique: when a question asks for the characteristics of a group, give features that every member has. "Animals have legs" loses the mark because worms and sponges have none.

✦

Practice

Try each question before reading the solution.

State four characteristics shared by all animals.

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Solution: Any four of: they are multicellular; they are eukaryotic; their cells have no cell walls; they are heterotrophic (they ingest food); most can move at some stage of life; most have tissues; most reproduce sexually.

A student says a coral is a plant because it never moves. Explain why the student is wrong.

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Solution: A coral cannot make its own food. It catches tiny animals with its tentacles, so it is heterotrophic. Its cells have no cell walls, and its young larva swims. Staying in one place does not make an organism a plant. Corals are animals in phylum Cnidaria.

State the type of symmetry of (a) a jellyfish, (b) a cow, (c) a sponge.

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Solution: (a) radial; (b) bilateral; (c) asymmetrical (no symmetry).

Arrange these ranks from the largest group to the smallest: species, phylum, kingdom, genus, class, family, order.

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Solution: kingdom → phylum → class → order → family → genus → species.

Summary

All animals are multicellular heterotrophs whose cells have no walls; most move and have tissues.
Organisms are placed in ranks: kingdom, phylum, class, order, family, genus, species.
Animals are divided using the backbone, body symmetry, number of body layers, body cavity, segmentation and type of skeleton.

Sources and credits

15.1: Features of the Animal Kingdom, from Concepts of Biology by OpenStax (Samantha Fowler, Rebecca Roush, James Wise), via LibreTexts Biology, licensed under CC BY 4.0. Adapted by Ganzaa.
15.2: Sponges and Cnidarians, from Concepts of Biology by OpenStax (Samantha Fowler, Rebecca Roush, James Wise), via LibreTexts Biology, licensed under CC BY 4.0. Adapted by Ganzaa.

The Rwandan examples, worked examples and practice questions are written by Ganzaa.

1What makes an animal an animal?

Worked example: is it an animal?

2The classification ladder

3Features used to divide the animal kingdom

4The phyla in this unit

Practice

What makes an animal an animal?

The classification ladder

Features used to divide the animal kingdom

The phyla in this unit