2.6 CLASSIFYING ANIMALS - INVERTEBRATES
Biologists classify animals into many large sets called phyla (singular phylum). Animals with
backbones belong to the phylum vertebrates. We will look at vertebrates in Module 2-7. Animals
in other phyla have no backbones and are called invertebrates. The keys below and on the next
page show three important phyla of invertebrates called molluscs, annelid worms and arthropods.
Molluscs are invertebrates like
A simple key to animals
snails and seashells. They have
soft bodies and usually carry hard
1. Animal has a backbone (Vertebrate)
GO TO MODULE 2.7
shells which they can hide inside.
No backbone (Invertebrate)
GO TO 2
There are about 50 000 different
kinds of molluscs. The largest are
2. Soft body and hard shell
MOLLUSC
squid which swim in the oceans.
Soft, round, long, thin body, segments
ANNELID WORM
Squid have long tentacles to catch
Hard covering, jointed legs (Arthropod) GO TO 3
their food and their shells are
3. Six legs
INSECT
inside their bodies. Some grow up
Eight legs
ARACHNID
to 45 m in
More than eight legs
GO TO 4
length. Many
molluscs
are
4. Four antennae, body segments all different CRUSTACEAN
edible and their
Many body segments and legs, all the same MYRIAPOD
shells are often
used to make
beautiful ornaments.
Annelid worms include earthworms, which live in the soil. There are about 9000 different kinds.
They all have soft, round, long, thin bodies divided into many segments, and they have no legs.
(Worms with flattened bodies, and worms with no segments, belong to quite different phyla. They
are not annelid worms). Earthworms help to make the soil more fertile and better for growing
crops. Leeches are annelid worms that feed by sucking the blood of larger animals, including
human beings. In the past, doctors used leeches to try to cure fevers! They believed that people
with fevers needed to loose some of their blood.
Arthropods have bodies with several segments, each covered with a hard outer casing. The
name arthropod comes from two Greek words, arthron (joint) and pod (foot). Arthropods have
bent legs with movable joints. They are divided into four main Classes: insects, arachnids,
crustaceans and myriapods.
Insects have six legs and three segments,
head, thorax and abdomen. All insects have
two antennae and most also have wings.
Insects are very successful animals. There
are more than 800 000 different kinds and
billions and billions of individuals. Although
each one is not very big, the total mass of all
the insects in the world is greater than the
mass of all the other animals put together!
Although some insects like houseflies,
cockroaches and mosquitos are pests, many
are useful to us, either directly or indirectly.
Arachnids have eight legs and only two
segments, head and abdomen. They have
no antennae and no wings. Common
examples are spiders, ticks and mites. Ticks
and mites live on the blood of larger animals
and may spread diseases. Many spiders are
useful because they catch and eat insects.
Crustaceans are like crabs and scorpions. They have more than
eight legs and four antennae. All their segments are different
from one another. Some of their legs may be different too, and
some may end in claws. Lobsters, crabs and prawns are eaten
by many people. Scorpions have a powerful sting in their tails. © D.G.Mackean
A scorpion
Myriapods include centipedes and millipedes. Their bodies have many, identical segments.
Centipedes have two legs on each segment, but millipedes have four legs on each segment.

1. What are (i) invertebrates, (ii) phyla, (iii) squid, (iv)
leeches, (v) myriapods? What does the word
myriapod mean?

2. What animal has eight jointed legs?
2 – 6a

3. State one way each of these is useful: (i) crab, (ii)
earthworm, (ii) spider, (iv) mollusc, (v) insect.

4. List all you can find out from the keys about
crustaceans.
A simple key to animals (invertebrates)
ANIMALS
animals have no backbone
INVERTEBRATES
animals have a soft body
and (usually) a hard shell
MOLLUSCS
eg snails, seashells
animals have a backbone
VERTEBRATES
(See Module 2.7)
animals have a soft, round, long, thin
body, divided into many segments
ANNELID WORMS
eg earth worms, leeches
animals have segments with hard
outer covers, and jointed legs
ARTHROPODS
animals have more than 8 legs
Snail
animals have 6 legs
INSECTS
eg house fly, beetles, butterflies
Earthworm
animals have 4 antennae;
body segments all different
CRUSTACEANS
eg scorpions, crabs, prawns
animals have 8 legs
ARACHNIDS
eg spiders, ticks
© D.G.Mackean
2 – 6b
animals have many identical
body segments and legs
MYRIAPODS
eg centipede, millipede