life on earth on the land

LIFE ON EARTH

ON THE LAND

THE DIAGRAM GROUP


Life On Earth: On the Land
Copyright © 2004 by The Diagram Group
Written, edited, and produced by Diagram Visual Information Ltd
Editorial director:

Denis Kennedy

Editors:

Bender Richardson White, Gordon Lee

Contributor:

John Stidworthy

Indexer:

Martin Hargreaves

Art director:

Roger Kohn

Senior designer:

Lee Lawrence

Designers:

Anthony Atherton, Christian Owens

Illustrators:

Julian Baker, Pavel Kostal, Kathleen McDougall, Coral Mula, Graham Rosewarne

Picture researcher:

Neil McKenna

All rights reserved. No part of this book may be reproduced or utilized in any form or by any means, electronic or mechanical, including
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Printed in the United States of America
EB Diagram

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This book is printed on acid-free paper.

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Contents
4 Introduction
THE LAND
6 Land
8 Climatic zones
10 Life on land
12 Mass extinctions

FOSSILS
14 Becoming a fossil
16 Dating fossils
18 Fossil fuels
ANIMALS WITHOUT
BACKBONES
20 Snails and worms
22 Spiders and scorpions
24 Millipedes and centipedes
26 Invertebrates
AMPHIBIANS AND REPTILES
28 Frogs and toads
30 Salamanders
32 Early reptiles
34 Rise of the dinosaurs
36 Dinosaurs in variety
38 Tortoises and tuataras
40 Lizards
42 Snakes
MAMMALS
44 First mammals
46 Egg-laying mammals
48 Ancient marsupials
50 Carnivorous marsupials
52 Plant-eating marsupials
54 Primitive placentals
56 Insectivores

58
60

62
64
66
68
70
72
74
76
78
80
82
84

Edentates
Primates
Rabbits and rodents
Squirrels
Mice and guinea pigs
Carnivores
Ancient carnivores
Modern carnivores
Primitive hoofed mammals
South American hoofed
mammals
Odd-toed hoofed
mammals
Even-toed hoofed
mammals
Ruminants
Elephants

BIRDS
86 Running birds
BIOMES
88 Tropical rainforest
90 Temperate woodland
92 Boreal forest
94 Tropical grassland
96 Temperate grassland
98 Desert
100 Mountains
102 The polar regions
104 Timeline
106 Glossary
109 Websites to visit
111 Index


4

Introduction

T

his book is a concise, illustrated guide to living things
that evolved on, and now inhabit, the land. Texts,
explanatory diagrams, illustrations, captions, and feature
boxes combine to help readers grasp important information.
A glossary clarifies the more difficult scientific terms for
younger students, while a list of websites provides links to

other relevant sources of additional information.
Chapter 1, The Land, looks at the living conditions that
animals face on land, and briefly reviews the course of
evolution among land animals. It also covers the topic of mass
extinctions throughout the Earth’s history.
Chapter 2, Fossils, tells how fossils are formed, how they are
dated, and how some are important to us as fuels.
Chapter 3, Animals without Backbones, gives an outline of
the main groups of invertebrates, both living and extinct, that
have taken to life on land. These include snails, worms, and
various groups of arthropods—animals with jointed legs.
Chapter 4, Amphibians and Reptiles, looks at the evolution of
these two groups of vertebrates, with examples of their
modern species.
Chapter 5, Mammals, is the longest section, examining the
ancient history of mammals, and then taking a closer look at
the various main groups, or orders, that make up this
important group of land animals.
Chapter 6, Birds, describes those birds that have forsaken
flying, to parallel the lifestyle of the running mammals.
Chapter 7, Biomes, looks at the various main habitats on
Earth, and how living things are adapted to them, with
examples of characteristic species.
On the Land is one of six titles in the Life On Earth series
that looks at the evolution and diversity of our planet, its
features, and living things, both past and present.
The series features all life-forms, from bacteria and algae to
trees and mammals. It also highlights the infinite variety of
adaptations and strategies for survival among living things,
and describes different habitats, how they evolved, and the

5

© DIAGRAM

communities of creatures that inhabit them. Individual
chapters discuss the characteristics of specific taxonomic
groups of living things, or types of landscape, or planetary
features.
Life On Earth has been written by natural history experts,
and is generously illustrated with line drawings, labeled
diagrams, and maps. The series provides students with a solid,
necessary foundation for their future studies in science.


6

Land

Biomes
These are areas of
similar climatic
conditions where
comparable types of
vegetation occur.

T

HE MAIN CONTINENTAL BLOCKS have

not always been arranged as they are
today. Over millions of years, slow
geological processes have gradually shifted the
“plates” of the Earth’s crust that carry the
continents. At various times in the past they
came together in different ways. For example,
Australia, Antarctica, and South America were
once connected. The geography of the past
had great influence on the evolution of various
animal and plant groups, and governed their
ability to spread.
Continental movements led to collisions that
sometimes crumpled the edges of the main continental blocks
slowly over millions of years. The Himalayas are the result of
India moving up from the south, and colliding with the main
block of Asia. The Andes have been thrust up where the Pacific
Ocean plate meets South America. The Himalayas and Andes
are “young” mountains, and contain many of the world’s

Land makes up about 30 percent
of the Earth’s surface. It is a vast
area of 57.5 million square miles
(149 million sq km). Two-thirds of
the land area in the present world
is in the Northern Hemisphere,
with Australia, most of South
America, part of Africa, and some
outlying islands of Asia in the
Southern Hemisphere.

Biomes
1

2

4

5




ON THE LAND THE LAND

highest peaks. Older mountains, such as those in
Scotland, have been worn down over hundreds of
millions of years, and are relatively low.
The average height of the continents above sea
level is about 2,756 feet (840 m), but there is a huge
variation in height from the tallest mountain,
Everest, at 29, 140 feet (8,882 m), to some parts of
the land, such as the shore of the Dead Sea, that
are as much as 1,299 feet (396 m) below sea level.
The land contains some areas, such as parts of
Australia, or Eastern Europe, with flat plains
stretching far and wide. The enormous variation in
landforms entails a similar variation in the
adaptations of the animals that live there. This is
one reason for the existence of a huge range of
animals in the modern world, and throughout the

many millions of years that animals have lived on
the land.
1
3

Tropical forest biome
Found near the equator
where conditions are
warm and very wet.

!

IT’S A FACT
The world’s continents, in
order of size (1,000 millions),
are as follows:
Continents
a Asia
b Africa
c North America
d South America
e Antarctica
f Europe
g Australia

Sq mi
17.5
12
9.5
7

5.2
4
3.0

The world’s continents
a

b

c
d

2 Desert biome
Very dry, often hot, and
with few plants. Covers
one fifth of the land.
3 Coniferous forest biome
Forests with long winters
and short summers.

6

4 Grassland biome
Warm or temperate, but
with not enough water
f0r trees to grow.
5 Temperate forest biome
Temperate, with enough
water for tree growth.
Many trees drop their

leaves in winter.
6 Tundra biome
Frozen for much of the
year with dwarf plants.

7

e
f

g

Sq km
44
31
24
18
13
10
8


8

Climatic zones

The world has a series of
marked climatic zones.
Equatorial regions are
warmed by sunlight

throughout the year.
Temperatures are lower
toward the Poles, where
sunlight has to penetrate a
thicker layer of atmosphere
before reaching the surface.

B

ECAUSE OF THE TILT of the rotating Earth, the
North and South Poles are plunged into
darkness in the middle of their winters. The
circulation of winds and ocean currents also affects
climate, but the basic pattern is clear. The tropics are
very warm throughout the year. At higher latitudes
there is a temperate zone, with warm summers but
cooler winters. At the highest latitudes, near the
Poles, there is a cold climate all year with, at most, a
brief summer during which some ice may melt.
It is not surprising that, in general, life is more
abundant and varied in the warm parts of the world,

January

Temperature belts in
January and July
Seasonal temperatures differ
more the greater the
distance from the equator.
(Temperatures are given

below in both Fahrenheit
and Celsius.)

July

0

Below -30 F

0

-34 c

0

0

-34 c to 1 c

0

0

-1 c to 10 c

-30 F to 30 F
30 F to 50 F
0

0

50 F to 70 F
0

0

70 F to 90 F
0

Over 90 F

0

0

0

0

0

0

10 c to 21 c
0

0

21 c to 32 c
0

Over 32 c


ON THE LAND THE LAND

9

Age of dinosaurs
At his time the world
enjoyed a warm climate.

as long as there is water available. The icy wastes of
Antarctica are least likely to support life, but there are
no places on the Earth entirely devoid of living things.
In mountains, temperature drops with height, so
different climatic zones are found at different heights.
On the highest mountains the summits tend to have an
Arctic feel.
The world can be divided into a number of “biomes,”
defined by climate and rainfall. Each has its own typical
vegetation and animal life, although the species may
not be the same on different continents.
But it was not always warm in the past. There is
evidence of great ice ages 445 million years ago, and
again about 300 million years ago. It is only recently,
geologically speaking, that the world emerged from an
ice age. Ice cover disappeared from North America
about 11,000 years ago. Some people believe that we
are now in a short, warm period within this ice age.


!

IT’S A FACT
It cannot be
assumed that today’s
climates are typical of
the past. For much of
the Cretaceous period
(144 to 65 million
years ago) the Earth
had a warm climate.
Even near the Poles it
was warm, so that
temperatures were far
more even across the
world than now. This
was the heyday of the
dinosaurs and
pterosaurs, and they
lived from the equator
to the Antarctic, even
though the polar
winters must have
been dark.

© DIAGRAM

Cretaceous period

At the end of this
period continents had
not reached their
present positions.


10

Life on land

First colonists of the land
Our early mammal
ancestors lived in the
shadow of the dinosaurs.

A

LITTLE BEFORE 400 million years ago plants
began to grow on land. Their ancestors were
probably green algae, but some of these new
land plants developed water-conducting tissue and,
soon after, supporting tissue. They could then grow
upward, rather than just form a crust or flat carpet.
Arthropods, the jointed-legged animals with hard
outer skeletons, were the first land animals. This is not
surprising, as their skin/skeleton could support them
out of water, and may also have provided some
protection against water loss. Their breathing
apparatus could be adapted to breathing air.
Some of these early land arthropods, such as

scorpions, were surprisingly similar to types still living
today. Few of the new land animals were adapted for
eating plants directly, although some could feed on decaying
vegetable matter. Most seem to have been predators.
Millions of years after these first colonists of the land, some
fishes took their first steps as tetrapods, four-legged
“amphibians.” It was many more millions of years before the
amphibians familiar to us today (frogs and salamanders)
evolved. Before these arrived on the scene, some tetrapods
became able to breed on dry land. By 300 million years ago
these so-called amniotes gave rise to others—reptiles and the
synapsid forerunners of mammals.
Mammals originated from synapsids at the time of
the dinosaurs. The earliest mammals did not
belong to groups you could see today.
The main groups of mammal we
recognize now—the egglaying monotremes, the
marsupials, and placental
mammals like us—did not
appear until much later.

Although living things
had been numerous and
diverse in the waters of
the world for hundreds
of millions of years, there
is little evidence of life on
land before about 400
million years ago. With
its bare, weather-beaten

surface, the land must
originally have been a
challenging place to live.


ON THE LAND THE LAND

Period

65–present Tertiary and
Quaternary
144–65

Who lived at that time?

Primate

Pantodont

Cretaceous
Mosasaur

206–144

Jurassic

Saurischian
Omithischian

Triassic

Crocodilian

290–248

Therapsid

Permian
Pelycosaur

Arthropod

354–290

Arthropod

Arthropod

Labyrinthodont

Silurian
Palaeophonus

490–443

Gymnosperm

Devonian
Fern ancestor

443–417

Conifer

Carboniferous
Protothyridid

417–354

Flowering plant

Saurischian

Bennettitalean

248–206

Perissodactyl

Rhyniophyte

Ordovician
Euthycarcinoid

543–490

Cambrian

2,500–543

Proterozoic

periods

Liverwort

No life on the land

No life on the land

© DIAGRAM

Millions of
years ago

11


12

Mass extinctions

Uintatherium (left)
A large mammal, it
evolved after the
extinction of the
dinosaurs.
Scutosaurus (right)
A plant-eating reptile, it
lived before the great
extinction 250 million
years ago.

N

ON-BIRD DINOSAURS disappeared completely
65 million years ago. Not only dinosaurs, but also
other reptiles, such as plesiosaurs in the sea and
pterosaurs in the air, vanished. Many fishes became
extinct, along with many invertebrates. Scarcely any
land animals bigger than wolf-size survived. The
disappearance of so many kinds of animals at the same
time is known as a mass extinction.
Why did so many animals disappear? We do not know
for sure, but there have been many theories. It appears
that a large asteroid hit the Earth at about this time, at
what is now the coast of Mexico. It would have thrown
up vast clouds of dust, and produced heat and fires. The Earth’s
climate was probably disrupted, possibly for many years.
On the other side of the world, in India, at about the same
time, there was a huge outpouring of lava that covered many
thousands of square miles (sq km), also affecting the climate.
Even before these events though, dinosaurs were declining.
Numbers had been dropping for 20 million years or so. Some
shellfish also disappeared millions of years before the layer of
rock that marks the “mass extinction.” Climates had been
changing as sea levels dropped, making continental interiors

For about 150 million
years, the dinosaurs were
the dominant large
animals on land. Diverse,

adapted for many ways
of life, and advanced for
their time, they seemed
destined to rule the
Earth forever.




13

Mass extinctions










ON THE LAND THE LAND

What was affected?

Possible cause?

445

Enormous cut in diversity of sea life

A great ice age, plus volcanic
activity

355

Trilobites; many kinds of fish; sponges

Global cooling; shallow water
areas much reduced

250

9o percent of species lost, including the
last trilobites

Great volcanic activity; smaller
seas; more extreme land climate

205

65 percent of marine species lost; over
30 percent of land vertebrates; most
land plants

Rising temperatures; asteroid
strikes

65

All non-bird dinosaurs; other large
reptiles; many
other species

Climate change; asteroid strikes;
volcanic activity

drier and harsher, and
creating land that animals
could cross between formerly
separate areas, increasing
competition. Perhaps the meteorite strike was
a sudden event that provided the final blow to
the declining groups of animals.
The mass extinction that saw the end of the dinosaurs
was not the only one in the Earth’s history, nor was it
the most catastrophic in terms of the percentage of
animals wiped out. From the rocks laid down
hundreds of millions of years ago come tantalizing
clues about factors that may have caused
the extinctions, but from this distance
in time we will probably never be sure.

Tyrannosaurus
This was one of the last
creatures to walk the
Earth before non-bird
dinosaurs disappeared
65 million years ago.

© DIAGRAM

Million years ago


14

Becoming a fossil

Amber (above)
Millions of years ago, this
fly was trapped in resin
and thus preserved.

O

CCASIONALLY, conditions are just right for the
remains of animals and plants to be preserved
in rocks. Even then, they may be destroyed by
geological processes, such as erosion, on the surface.
But some fossils remain preserved in rocks for millions,
or even hundreds of millions, of years, and may be dug
up to give us an insight into life long ago.
Small, soft-bodied animals stand least
chance of preservation. Animals in
water may die, fall to the bottom, and
be buried in mud. Their skeletons or
shells may not decay. Covered
eventually by a great thickness of

sediment, new minerals may
gradually replace bone or shell,
making a hard replica of the original.
Sometimes the original hard parts
are instead dissolved away by acids
seeping through the sediment, leaving
just a hole, but one that retains the shape
of the animal that was formerly there.
On land, becoming a fossil is even rarer than in
water. Many land animal fossils are preserved because
they died in, or near, water and have been carried into mud and
preserved. On land, most animals are consumed, or decay
rapidly. This is particularly true of forest animals. Sometimes
burial by wind-blown sand or dust, or covering by volcanic ash,

The great majority of
animals and plants that
die do not become fossils.
They are eaten by other
organisms, or decay
away completely.

Fossilization
Burial under water is
the commonest way
of becoming a fossil.
An animal
dies and
sinks to the
bottom.

The animal is
buried in the
mud.

Layers of
mud cover
the animal.

Layers of
mud are
crushed into
hard rock.

The rock
wears away
and the fossil
is exposed.




ON THE LAND FOSSILS

can lead to fossilization. Small animals can be
trapped in tree resin. The resin itself becomes
fossilized as amber, with insects or spiders
trapped within it.
Relatively recent animals, from thousands
rather than millions of years ago, may also be

preserved by mummification in a dry climate,
or by freezing in a cold one.

§

STRANGE
BUT TRUE
It is not just
animals’
bodies that
become
fossils.
Burrows
can also be
preserved in
rocks. Some A dinosaur’s footprint
of the earliest land fossils are
burrows. It is not always easy to
guess what made them.
Footprints and trackways can
also be preserved in mud. If
they can be matched to a
particular animal, we can learn
how, and sometimes how fast,
an animal moved millions of
years ago. Fossil droppings,
called coprolites, are also found.
Again, if their maker can be
identified, it may provide
information about eating

habits. Even eggs and nests can
be buried in the sand to
become fossils.

A fossil
dropping

© DIAGRAM

Fossilized tetrapods in New Mexico
These “amphibians” died 200 million years ago, and
were preserved in the mud of a drying-up pond.

15


16

Dating fossils

S

OMETIMES, areas
of rock are
deformed or folded
upside-down over a
certain area, but, as long
as geologists recognize
Archaeopteryx
what has happened,

This fossil comes
older and younger layers
from the Jurassic
can be traced with precision.
period (150 million
From studies over large areas of the
years ago).
world, a picture of the sequence of rocks
through the ages was built up. The major geological
periods, such as the Cretaceous, were recognized by the
rock formations laid down in them. It was possible to make
estimates of the time it took for the huge thicknesses of
Brown limestone
sediments to be laid down, and so calculate the probable dates
and duration of these periods.
Fossil oyster bed
Widespread animal species that existed for a limited time
aid the recognition of sediments of the same period, even if far
Brown limestone
apart. Detailed detective work allowed scientists to put most
rocks they found in sequence. But until the properties of
Hard sandstone
radioactive elements were discovered, there was no way of
putting a date on a rock with any degree of certainty. Using
radioactive isotopes, absolute dating is possible, at least for
Green shale
igneous rocks (those formed by volcanic activity).
Minerals in these rocks may contain a radioactive element
such as uranium-235. Atoms of this element “decay,” losing
part of their nucleus, and turn into lead-207 at a steady

rate. Half of a sample of this uranium will turn into lead
Dark shale
in 713 million years. If you measure the relative
amounts of uranium-235 and lead-207 in the rock,
you can establish an absolute measure of how
Green shale

The simplest way of
establishing when a fossil
lived is to examine the
layer of rock in which it
was found. Layers of
sedimentary rock lie on
top of one another. In
undisturbed rocks, upper
layers are younger than
lower ones.

Paleocene sandstone
and tertiary basalt in
Greenland caption to
come

Fallen rocks

Dating fossils
Layers in sedimentary rocks allow us to
tell the relative dates of fossils. Lower
layers are older than upper ones.

ON THE LAND FOSSILS

Geological timescale (dates given in millions of years)
Eras
CENOZOIC
65–Present

Periods
Quaternary
1.8–Present
Neogene

or Late Tertiary 23.8–1.8

MESOZOIC
248–65

Paleogene
or Early Tertiary
65–23.8

Different aged strata (left)
By virtue of the relative position of strata, the
comparison of fossils, and radiometric dating, the
ages of strata, and their position in the geological
timescale, have been established.

Epochs
Holocene (Recent)

0.01–Present
Pleistocene
1.8–0.01
Pliocene
5.3–1.8

much time has elapsed since the rock
was originally formed.
However, measurement and
calculation is rarely as easy as this
suggests. Different radioactive elements
have different “half-lives” of decay, and
are useful for different periods of the
Earth’s history. Where igneous rocks
occur between layers of sediment, they
can give a date to the sediment.
Nowadays, the ages of enough rocks
have been calculated for us to have a
clear idea of the likely date of most
fossils, although there may still be room
for some adjustments.

Miocene
23.8–5.3
Oligocene
34–23.8

Cretaceous
144–65

Eocene
55–34
Paleocene
65–55

PALEOZOIC
543–248

Jurassic
206–144
Triassic
248–206

Permian
290–248

Carboniferous
354–290

Devonian
417–354
Silurian
443–417

17

Radiometric dating

By radiation potassium-40 loses half its mass every
1,310 million years (one half-life). Thus a sample‘s

potassium-40 content can indicate its age,
A Original sample

1/1

A

B After 1.3 billion years (one half-life) half remains.
C After 2.6 billion years (two half-lives) one quarter
remains.
D After 3.9 billion years (three half-lives) oneeighth remains.

Ordovician
490–443

B

E After 5.2 billion years (four half-lives) onesixteenth remains.

1/2

Cambrian
543–490

C

1/4

D

1/8
1/16
0

1

2

3

4

E
5


18

Fossil fuels

Coal, and underground oil and gas
deposits, are all fossil fuels. They
formed long ago from the remains
of dead organisms. These remains
were concentrated into a form
which is useful to people.

C

ONDITIONS HAVE NOT always been

suitable for the formation of fossil fuels.
Most of the deposits come from just a
few of the many geological periods. No
deposits of significance are being laid down
now. Even though there are vast quantities in
the ground, they are being used. A time will
come when it is no longer
possible to power civilization
from fossil fuels.
Much of the best coal was
formed over 300 million years
ago. A geological period,
known as the Carboniferous
(354–290 million years ago),
was named because of the
abundance of carbon—coal—
in its rocks. During this
period, plenty of swamp
forests grew on low ground,
with trees up to 100 feet (30
m) tall. They were club
mosses and horsetails. Their
present-day relatives are
much smaller. When they
died, they fell into acidic
water and were partly
preserved, instead of
decaying away.

Legacy

The swampy forests of 300 million
years ago are the source of much of
the good quality coal in use today.




ON THE LAND FOSSILS

From time to time, seawater flooded some of the lowlying forests, and sediment was dumped on top of the plant
remains, compressing them. Then the water level dropped
again, plants grew, and the cycle repeated itself. Most of the
time there was forest, with briefer periods when sediment
was dumped. Even so, the coal seams are thin compared to
the sediments surrounding them.
Oil is also formed from the remains of living things. Often
these were tiny sea organisms that died and accumulated
at the bottom of still water. The carbon compounds in their
bodies seeped down and were trapped in the rock.
Deposits of natural gas are often found above oil
reserves, derived from the same creatures. Particular
limestone formations are associated with oil, as are some
salt deposits. More than half of the world’s oil—in the rich
deposits of the Middle East and the Gulf of Mexico—
started forming during the second half of the time of the
dinosaurs. Texan oil comes from more than 100 million
years earlier.

19

§

STRANGE
BUT TRUE
It is estimated that a
thickness of about
100 feet (30 m) of
plant remains must
be compressed to
provide a 3 foot (1 m)
seam of coal. It
would have taken
5,000 years, or even
more, to produce
these plants.

© DIAGRAM

Where they are
This map shows some of
the main areas in the
world where fossil fuels
are found.


20

Snails and worms

Worms are some

of the oldest
animal types
but, as they are
soft-bodied, they
rarely fossilize.

Body plan (below)
Earthworms have a
simple, segmented body
plan, but are still
successful soil animals.

Body wall

S

OMETIMES TRACES OF THE soft bodies of worms, or
fossils of hard jaw parts or burrows, are found. But
nearly all are remains of marine worms. Land worms are
even less likely to become fossils. Gastropod mollusks (snails)
are known from over 500 million years ago, but these first
snails were sea creatures. Far from fading out, snails have
been increasingly successful over the last 100 million years or
so. Shells fossilize well, but most gastropod fossils are from
either sea or fresh water. A few are land snails related to those
found in our gardens.
Earthworms are the most familiar worms on land. They
burrow in soils all over the world. They have bodies divided
into rings called segments, sometimes as many as 200 or
more. Many segments are similar, but the front segments

contain the mouth and small “brain,” hearts,
and reproductive organs. Most worms eat
Mouth
pieces of dead plants in the soil, or rotting
leaves brought from the surface. Their

Segments

Anus


?

DID YOU KNOW?
Most earthworms are 12 inches
(30 cm) long or less, but in western North
America there are bigger species. In South
Africa and Australia there are worms that
grow to over 10 feet (3 m) in length!


ON THE LAND ANIMALS WITHOUT BACKBONES

21

activities in burrowing and
churning the soil help to
enrich and aerate it. Worms are also an important food for
Roundworm (above)
This type of worm often

many other kinds of animal.
lives inside plants or the
Even more common than earthworms, but far less visible,
bodies of other animals,
are roundworms. These have a totally different body plan,
including humans.
without segments, and with smooth tough skins. They are
pointed at both ends, and the outside of the body is almost
featureless. Although some are large, most are tiny or even
microscopic. Some live freely in the soil,
but many live as parasites inside the
Digestive
bodies of other animals and plants.
Intestine
gland (liver)
Roundworms are largely hidden.
Lung
Snails are also unsegmented. A snail
Kidney
moves on a large muscular “foot.” Glands Salivary gland
produce slime to smooth its way. The
digestive system and other organs are
Heart
tucked inside a hard, coiled shell. The
snail retreats into this for protection
from enemies. The head bears sense
Mouth
Anus
Ureter
Stomach

organs. In the mouth is a tongue covered
Snails
with many horny teeth that the snail
Like most mollusks, snails
uses to rasp at plants as it feeds.

© DIAGRAM

have a complex internal
structure (above), and
shells made of calcium
carbonate (below).


22

Spiders and scorpions

Eotarbus (above)
This was one of the first
creatures to inhabit the
land about 415 million
years ago.
Bird spider (below)
This bird-eating spider
measures about two and
a half inches (6 cm) in
length, and comes from
Panama.

T

HE FIRST FOSSILS of actual animals date from as
much as 415 million years ago, and include a
scorpion and a tiny arachnid named Eotarbus. This
was not in fact a spider, but it did resemble present-day
spiders and mites.
Around 395 million years ago, a place called Rhynie in
Scotland was full of volcanic springs, around which grew
miniature forests of primitive plants. They, and the animals
living in them (including mites and relatives of Eotarbus),
became fossilized. In some, so-called book lungs are
preserved; these are folded structures that are found in living
scorpions (and some spiders). They are used for breathing air,
and were probably modified from gills of aquatic ancestors.
The earliest known “true” spider was found in rocks from
the eastern United States about 375 million years ago. It had
fangs with poison glands, and also a spinneret—an organ that
feeds out the silk of the web. It looks as though, even then,
spiders were spinning webs or trap lines.
Spiders, scorpions, and mites are all arachnids. These
animals have segmented bodies (though the segmentation is
not obvious in many spiders) and eight
walking legs. In front of these, other pairs
of legs are turned into jaws. In scorpions
they include a pair of large
pincers for seizing prey. In

Tracks and burrows
suggest that there may

have been animals on
land as long as 450
million years ago.



§


ON THE LAND ANIMALS WITHOUT BACKBONES

Scorpion (left)
While seizing
prey with their
pincers, scorpions
may also use the
sting in their tail
to subdue it.

spiders there are downward-stabbing fangs in the more
primitive types, or pincerlike fangs in the more advanced. All
spiders produce venom from their fangs, but although lethal
for small prey, few kinds are dangerous to humans. Scorpions
have a venom gland attached to the sting at the tip of the tail.
Again, the venom kills small animals, but few scorpions are a
threat to us.
Both spiders and scorpions have a long history, but nowadays
there are only about 1,200 kinds of living scorpion. More
than 35,000 kinds of spider have been named.
There are other types of arachnid including “false

scorpions,” sun spiders, and at least 30,000 kinds of
mite. There are probably many more tiny mites in
existence, many living on, or else inside, other living
organisms, but we have a tendency to notice
mainly those that cause
discernible diseases in
both domestic animals
and plants.

Dust mite (right)
Although this creature is
microscopic in size, some
people are allergic to it.

23

False scorpion
These are tiny
arachnids that are
found in leaf litter.


24

Millipedes and centipedes

For posterity (above)
An ancient millipede is
trapped in amber.

Arthropleura (above)
This millipede lived 300
million years ago, and
was 6 feet (2 m) long.

O

N LAND, fossil burrows from 450 million
years ago have been interpreted as
belonging to millipedes, and pellets of
plant remains from 410 million years ago could be
millipede droppings. Remains of centipedes from
over 400 million years ago, and millipedes from at
least 375 million years ago, confirm them as among the
earliest-known land animals. Giant relatives of millipedes 6
feet (1.8 m) long and 20 inches (50 cm) wide trundled across
the land 315 million years ago, leaving marks resembling
railway tracks.
Both centipedes and millipedes have a head with specialized
segments carrying jaws and sense organs, but behind the head
the segments are mostly very similar to one another, and bear
pairs of walking legs. These animals hatch from an egg with
only a small number of segments, and add more until they
reach adulthood.
With a few exceptions, the 10,000 or so species of millipede
are plant eaters, feeding on dead or dying vegetation. They
tend to tunnel into the earth, or else push through the
leaf litter. Even after millions of years of evolution, they
have not developed a waterproof coat to the skin, and
are usually found in damp places. Most millipedes have simple

Centipedes and millipedes are
ancient animal types, whose
relatives lived in the sea 500
million years ago.

Millipede head (below)
Jaws that have adapted for
nibbling plants remain.

Millipede head

Eye

Antenna
Walking legs