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102 Nature

ECHINODERMS

6,000 species include starfish,

sea urchins, and sea cucumbers.

REPTILES

8,000 species include crocodiles,

lizards, snakes, and turtles.

FISH

29,000 species include bony fish,

sharks, rays, and lampreys.

BIRDS

9,500 species include eagles, gulls,

parrots, ducks, and perching birds.

AMPHIBIANS

6,000 species include frogs, toads,

newts, and salamanders.

CNIDARIANS

10,000 species include corals,

sea anemones, and jellyfish.

SPONGES

10,000 species include tube

sponges and glass sponges.

INSECTS

800,000 species include

beetles, flies, and ants.

MOLLUSCS

70,000 species include slugs,

snails, mussels, and squid.

ANNELID WORMS

9,000 species include

earthworms and leeches.

WHAT DOES THE SKELETON DO?

The skeleton is a strong frame that supports the body

and anchors the muscles. Bones such as ribs protect

the heart and other delicate parts, and the skull

shields the brain. Most vertebrates have bony

skeletons, but shark skeletons are rubbery cartilage.

HOW MANY LIMBS DO VERTEBRATES HAVE?

Most vertebrates apart from fish have four limbs. In

birds and bats, the front limbs evolved (developed)

into wings for flying. A fish’s limbs are its fins, which

vary in number between species. Snakes evolved from

four-legged ancestors millions of years ago.

DO INVERTEBRATES’ EXOSKELETONS GROW?

The hard exoskeletons of insects and creatures such

as crabs do not grow with the rest of the body. As the

animal grows, its exoskeleton gets too tight – so it

sheds it from time to time. Underneath is a new,

slightly bigger, and looser case. The animal pumps

itself up with fluid before the case has time to harden.

1 PYTHON SKELETON

Like all vertebrates, snakes have

a skull, backbone, and ribs. The

backbone is made of many

individual bones called vertebrae.

2 VERTEBRATE GROUPS

Fish are the largest vertebrate

group, with more species than all

the other groups put together.

Invertebrates

About 95 per cent of all animals are

invertebrates – animals without

bones – and many are tiny

or even microscopic.

Vertebrates

All vertebrates have an inner skeleton, including a skull,

backbone, and ribs. These complex animals vary in size

from less than 1 cm (^2 ⁄ 5 in) to over 30 m (100 ft) long.

MAMMALS

5,000 species include tigers, rodents,

bats, whales, and primates.

2 INVERTEBRATE GROUPS

Invertebrates include more than

30 different phyla (major groups)

of animals. Some of the most

important ones are shown here.

Anus

expels waste

Tube feet

pass food to the

mouth and help

the animal move

Intestine

digests food

INSIDE A SEA URCHIN 3

Like most invertebrates, sea

urchins have relatively complex

internal organs. Food is broken

down using five teeth contained

in a central structure known as

Aristotle’s lantern, before passing

into the intestine.

Hard exoskeleton

Haemal system

transports blood

around the body

Muscles

operate the

movement

of the teeth

Tooth

HOW DO ANIMALS SURVIVE WITHOUT BONES?

Insects, crustaceans, and many other invertebrates

have a hard outer case called an exoskeleton. This

protects them against knocks and predators, and

prevents them drying out. Slugs, leeches, and jellyfish

have soft bodies and no exoskeleton. The pressure of

fluids inside their bodies maintains their shape.

FIND OUT MORE. Crustaceans 109 • Insects 110–111 • Molluscs 106

FIND OUT MORE. Animals 96–97 • Skeleton 130–131

invertebrates

vertebrates