they also have joints, or bendable sec-
Hiking in the mountains is a
good way to get exercise.
172 Exoskeleton BRITANNICA STUDENT ENCYCLOPEDIA
tions. These joints allow the animals to
move easily. The exoskeletons of land
animals also have small breathing holes,
which are called spiracles.
As animals with exoskeletons age, their
soft inside parts grow. But their
exoskeletons do not grow. When an
animal’s soft body gets too big for the
exoskeleton, the exoskeleton splits open
and falls away. This process is called
molting. The animal’s body then forms
a new exoskeleton. The animal has no
protection while the new exoskeleton is
forming.
Many of the invertebrates known as
mollusks have a type of exoskeleton
called a shell. Mollusks include clams,
oysters, scallops, conchs, mussels, and
snails. Their shells are made of a substance
called calcium carbonate. Mollusks
with shells do not molt. As the
animals grow, the shells grow, too, along
the edges.
#More to explore
Insect • Shell
Exploration
People throughout history have gone
exploring to learn about unknown
places. By the beginning of the 21st
century most of Earth’s surface had been
explored. Exploration, however, continues
in new directions—thousands of feet
below sea level and many miles into
space.
Exploration of Earth’s Surface
The first known explorer was Hannu, an
Egyptian who lived in 2750 BC. He
brought riches back from what are now
Ethiopia and Somalia. Many other great
explorers followed. In the 300s BC Alexander
the Great of Macedonia (near
Greece) made colonies of the lands he
A cicada breaks free from the exoskeleton it
has outgrown.
A monument in Lisbon, Portugal, honors
Portuguese discoverers. Throughout history,
explorers have been admired for their
courage and spirit.
BRITANNICA STUDENT ENCYCLOPEDIA Exploration 173
explored, as far away as India. In the AD
1200s Marco Polo traveled from Italy to
China.
A great age of European exploration by
sea began in the 1400s. Portuguese
explorers sailed along the coasts of
Africa, Arabia, and India. In 1492
Christopher Columbus sailed west and
landed in the Americas. In 1522 Ferdinand
Magellan’s ship completed the first
voyage around the world. In the 1700s
the British explorer James Cook reached
Australia, Hawaii, and other islands in
the Pacific Ocean.
Many explorers traveled inland from the
coasts to seek riches, to build settlements,
or to spread Christianity. In the
process, they met—and often fought
with—native peoples. In the 1500s
Spanish explorers called conquistadors
conquered much of Mexico and South
America. Between 1804 and 1806 the
Lewis and Clark Expedition explored
the western parts of North America.
Later in the 1800s David Livingstone
and Sir Henry Morton Stanley explored
parts of Africa that Europeans had never
seen.
Explorers first reached Earth’s poles in
the 1900s. Robert E. Peary and Matthew
Henson were the first people at the
North Pole, in 1909. The first explorer
at the South Pole was Roald Amundsen,
in 1911.
Undersea and Underground
Exploration
In 1960 DonaldWalsh and Jacques Piccard
reached the deepest part of the
ocean, 35,800 feet (10,912 meters)
down. They were in a craft called a
bathyscaphe. Even now, much of the
ocean remains unknown. Scientists are
still discovering new forms of undersea
life.
A huge unexplored region lies beneath
the ground as well. Russian scientists
drilled the deepest hole into Earth
between 1970 and 1989. It was 7.6
miles (12.2 kilometers) deep.
Space Exploration
A human traveler first explored space in
1961. In that year the Soviet cosmonaut
Yuri Gagarin orbited, or traveled
around, Earth in a spacecraft. On July
20, 1969, the U.S. astronauts Neil Armstrong
and Edwin Aldrin became the
first humans on the Moon.
Thanks to modern technology, places
where no people have gone can still be
Most of Earth’s
surface has
been
explored. But
there are
many caves
just below the
surface that
are still unexplored.
A crane lowers a craft called a bathyscaphe
into the water in 1959. Auguste
Piccard and his son Jacques invented the
craft to explore the deep sea.
174 Exploration BRITANNICA STUDENT ENCYCLOPEDIA
explored. Unmanned spacecraft are
operated by radio. These space probes
can travel deep into space without having
to return to Earth. Space probes
have sent back pictures and other information
about the planets and other parts
of the solar system.
#More to explore
Americas, Exploration and Settlement of
the • Polar Exploration • Space
Exploration
Extinction
#see Animals, Extinct.
Eye
Humans and other animals use their
eyes to see. When people see an object,
they actually see light reflecting, or
bouncing, off that object. This light
enters the eye. The eye changes the light
into electrical signals, which travel
through the optic nerve to the brain.
The brain interprets, or reads, these signals
as an image, or picture, of the
object.
The Human Eye
Humans have two eyes. Each eyeball sits
in a socket, or opening, in the skull. The
skull bone protects the eye on the sides
and back. The eyelid protects the front
of the eye. By blinking, eyelids also
move tears across the eye. Tears keep
