B. Diversity of Living Things · Diversity of Living Things

The Animal Kingdom: Body Plans, Invertebrates and Vertebrates

B3.3 B2.2 B2.1About 75 minutes

Learning goals

I can list the characteristics shared by all animals.
I can classify animals using symmetry, tissue layers, body cavity and segmentation.
I can describe the main invertebrate phyla and the chordate features, with Canadian examples.
I can distinguish vertebrates from invertebrates and compare the vertebrate classes.

Words to know

radial / bilateral symmetry
Body parts arranged around a central axis like a pie / a body with matching left and right halves, usually with a head end.

cephalization
Concentration of nerve tissue and sense organs at the head (anterior) end.

germ layers
The embryo's tissue layers: ectoderm (outer), mesoderm (middle) and endoderm (inner).

coelom
A fluid-filled body cavity completely lined with mesoderm, which holds and cushions the organs.

segmentation
A body made of repeated units (segments), as in earthworms, insects and the vertebrae of chordates.

exoskeleton
A hard outer skeleton, made of chitin in arthropods, that supports and protects the body and must be moulted to grow.

notochord
A flexible rod along the back of a chordate embryo; in vertebrates it is replaced by the backbone.

ectotherm / endotherm
An animal whose body temperature depends mainly on its surroundings / an animal that keeps a steady body temperature using heat from its own metabolism.

More than just backbones

Ask someone to name an animal and they will probably say a dog, a moose or a bird. But about 95% of described animal species are invertebrates, animals without backbones: insects, spiders, worms, snails, clams, jellyfish and sponges. A single Ontario backyard can hold hundreds of invertebrate species.

How do biologists bring order to this variety? They look at body plans: the basic architecture of an animal's body.

Remember from Grade 6

In Grade 6 you sorted animals into vertebrates and invertebrates and learned that arthropods have an exoskeleton, and that some animals are ectotherms and others endotherms. This lesson adds the features biologists use to classify animals into phyla.

What all animals share

Multicellular eukaryotes with no cell walls.
Heterotrophs that ingest food and digest it inside the body. (A few parasites, such as tapeworms, have no gut and absorb food through their body surface instead.)
Most can move at some stage of life (even sponges and barnacles have swimming larvae).
Most reproduce sexually; the embryo goes through a hollow ball stage called a blastula.
All except sponges have true tissues, including nerve and muscle tissue.

Four features of body plans

Symmetry. Sponges are asymmetrical. Cnidarians (jellyfish, hydras) have radial symmetry, which suits animals that stay put or drift and meet food from every direction. Most animals have bilateral symmetry, with a head end that leads (cephalization), which suits animals that move forward.
Tissue (germ) layers. Cnidarians develop from two layers; most animals from three: ectoderm, mesoderm and endoderm.
Body cavity. Flatworms are solid (acoelomate). Roundworms have a pseudocoelom. Most animals have a true coelom, lined with mesoderm, giving organs room to grow and move.
Segmentation. Repeated segments appear in annelids (earthworms), arthropods and chordates (our vertebrae and ribs). Segments allow specialized body regions and flexible movement.

Three kinds of symmetry. A sponge has no symmetry; a jellyfish has radial symmetry around a central axis; a flatworm has bilateral symmetry with a head end.

Cross-sections of three body plans. An acoelomate is solid between gut and skin; a pseudocoelomate has a cavity between the mesoderm and the gut; a coelomate has a cavity (coelom) completely lined by mesoderm.

Phylum	Symmetry	Body cavity	Key features	Canadian examples
Porifera (sponges)	none	none (no true tissues)	pores; water pumped through body; filter feeders	freshwater sponges in Ontario lakes
Cnidaria	radial	none (2 layers)	stinging cells; tentacles; one gut opening	hydra in ponds; moon jellyfish off both coasts
Platyhelminthes (flatworms)	bilateral	acoelomate	flat; one gut opening; some parasites	planarians in streams; tapeworms
Nematoda (roundworms)	bilateral	pseudocoelom	round, unsegmented; gut with two openings	soil nematodes (millions per square metre)
Mollusca	bilateral	coelom	soft body, muscular foot, mantle; most have a shell	garden snails, freshwater mussels, squid
Annelida (segmented worms)	bilateral	coelom	ring-like segments; bristles in many	earthworms, leeches
Arthropoda	bilateral	coelom	chitin exoskeleton; jointed legs; segmented	monarch butterflies, spiders, crayfish
Echinodermata	radial as adults (bilateral larvae)	coelom	spiny skin; tube feet; water vascular system	sea stars, sea urchins (marine only)
Chordata	bilateral	coelom	notochord, dorsal nerve cord, pharyngeal slits, post-anal tail	lancelets, lampreys, loons, beavers, humans

The nine major animal phyla, from simplest to most complex body plans.

Arthropods: the most diverse phylum

More than three-quarters of described animal species are arthropods. Their main classes:

Insects: 3 body regions (head, thorax, abdomen), 6 legs, 1 pair of antennae, often wings.
Arachnids (spiders, ticks, mites): 2 body regions, 8 legs, no antennae.
Crustaceans (crayfish, lobsters, pill bugs): 2 pairs of antennae, many legs, mostly aquatic. The pill bugs in your garden are land crustaceans.
Myriapods: many segments; centipedes have 1 pair of legs per segment, millipedes 2 pairs.

Chordates and vertebrates

At some point in their lives, all chordates have four features: a notochord, a dorsal hollow nerve cord, pharyngeal slits or pouches, and a post-anal tail. In vertebrates, the notochord is replaced by a backbone of vertebrae that protects the spinal cord.

The vertebrate classes:

Jawless fish (lampreys; the sea lamprey is an invasive parasite in the Great Lakes).
Cartilaginous fish (sharks, rays): skeleton of cartilage.
Bony fish (lake trout, walleye): bony skeleton, gills, swim bladder.
Amphibians (frogs, salamanders): moist skin; most lay eggs in water; larvae usually aquatic.
Reptiles (turtles, snakes): dry scaly skin; amniotic eggs with shells.
Birds (common loon): feathers, hollow bones, endotherms.
Mammals (beaver, moose): hair, mammary glands, endotherms.

Fish, amphibians and reptiles are ectotherms; birds and mammals are endotherms. (Cladistics shows that birds are a branch of the reptile lineage, closest to crocodiles, though the traditional classes are still widely used.)

Worked example

Let's classify animals into phyla

Use body-plan features to name the phylum: (a) a soft-bodied animal with a muscular foot and a coiled shell, found on a wet garden path; (b) a long, round, segmented worm with tiny bristles; (c) an animal with 8 jointed legs and 2 body regions; (d) a small pond animal with a tube-like body, tentacles with stinging cells and radial symmetry.

Look for the most telling feature first: shell and foot → molluscs; segments → annelids or arthropods; jointed legs → arthropods; stinging cells → cnidarians.
(a) Muscular foot + shell → Mollusca (a snail).
(b) Segments + bristles, no legs → Annelida (an earthworm).
(c) Jointed legs → Arthropoda; 8 legs and 2 body regions → class Arachnida.
(d) Tentacles with stinging cells, radial → Cnidaria (a hydra).

(a) Mollusca, (b) Annelida, (c) Arthropoda (arachnid), (d) Cnidaria.

Worked example

Let's compare: is a spider an insect?

A student calls a spider an insect. Use evidence to decide.

Insects: 3 body regions, 6 legs, 1 pair of antennae, often wings.
Spiders: 2 body regions (cephalothorax and abdomen), 8 legs, no antennae, no wings.
Both have jointed legs and exoskeletons, so both are arthropods, but they differ in class-level features.

No. A spider is an arachnid, not an insect; both are arthropods.

Just for fun

A funny-but-true fact: most earthworms in Ontario's gardens are not originally Canadian. Glaciers wiped out earthworms across much of Canada, and the common species here today arrived from Europe, likely in soil carried by settlers. Some forests are still adjusting to these wriggly newcomers.

Hands-on activity

Invertebrate survey under logs and leaf litter

You need: garden gloves, a white tray or a light-coloured plate, a spoon, a magnifying glass, a notebook, and a grown-up. Do this in a backyard, garden or a park where it is allowed.

Choose 3 spots: under a log or flat stone, in leaf litter, and in garden soil. To lift a log or stone, roll it toward you so it is between you and anything underneath, and put it back exactly as it was.
Gently spoon or shake a handful of litter or soil onto the tray. Watch for movement for 2 minutes.
For each animal, count its legs and body regions, look for a shell, segments or antennae, and decide its phylum (and class for arthropods). Record a tally.
Make one labelled biological drawing of an invertebrate (for example, a pill bug: label antennae, segments, legs).
Return all animals and materials where you found them. Wash your hands afterwards.

What to look for: common finds are earthworms (Annelida), snails and slugs (Mollusca), pill bugs (crustaceans), millipedes and centipedes (myriapods), spiders and mites (arachnids), and beetle larvae, ants and springtails (insects). Note which phylum was most common. Check for ticks after outdoor work; don't handle centipedes (some can pinch or bite).

Your turn

List four characteristics shared by animals.

Why does bilateral symmetry suit an animal that moves forward?

What is a coelom, and why is it useful?

Name the four features shared by all chordates at some stage.

Give two differences between insects and arachnids.

Which phyla show segmentation?

Is a sea star a fish? Explain.

More practice: warm-up

Which phylum do sponges belong to?

What is an exoskeleton made of in arthropods?

What kind of symmetry does a jellyfish have?

Name one endothermic vertebrate class.

Which phylum includes snails and clams?

More practice: on your own

A pill bug has 14 legs, 2 pairs of antennae and gills. Is it an insect? Classify it.

Why are tapeworms classified as flatworms and not roundworms?

Explain why an arthropod must moult, and one risk of moulting.

Humans don't have a tail or gill slits. Why are we chordates?

Compare ectotherms and endotherms in a Canadian winter.

Challenge

Sea star larvae are bilaterally symmetrical, but adults are radial. What does this suggest about their evolutionary history, and where would you place echinoderms relative to cnidarians?

Explain how a coelom and segmentation together give earthworms an advantage for burrowing.

About 95% of described animal species are invertebrates. Give two reasons why arthropods in particular are so diverse.

Check your understanding

Quiz

Which feature is shared by ALL animals?

An animal with no body cavity between its gut and body wall is:

Which animal is an arachnid?

Which phylum has a chitin exoskeleton and jointed legs?

Which is NOT a chordate feature?

Reptiles differ from amphibians because reptiles:

Radial symmetry is found in:

For parents and teachers

Big idea (B3.3, B2.2): animals are classified by body plan: symmetry, germ layers, body cavity and segmentation. Students learn nine major phyla, the chordate features and the vertebrate classes, and distinguish vertebrates from invertebrates.

How to help: The leaf-litter survey is a great family activity. Ask your teen to count legs and body regions before naming anything.

Common difficulties:

Calling all small crawling animals "bugs" or insects. Spiders, pill bugs and centipedes are not insects.
Thinking chordate = vertebrate. Lancelets are chordates without backbones.
Mixing up the three body-cavity types.

Safety: wear gloves, roll logs toward you, return everything, check for ticks, and wash hands.

Sources: phylum features from standard zoology texts; non-native earthworms in Canada from the Canadian Wildlife Federation and Natural Resources Canada; sea lamprey from the Great Lakes Fishery Commission.