Rubrics for Lesson 9
Rubrics for Lesson 9
Dealing with Gravity
Animals who climbed out of the water found that they
needed to learn to deal with gravity. This required the use of
more energy and the development of stronger muscles, bones, and
Gravity and Structural
In the water, many animals are almost
"weightless" because the water supports their bodies.
When animals crawled out onto the land they were suddenly seized
by gravity. What a surprise that must have been! Tiny, crab-like
animals, the ancestors of the insects, kept their small size
and light-weight exoskeletons. Gravity was not a major problem
for them, and many eventually grew wings and learned to fly.
Larger animals with internal skeletons
had to adapt to gravity. This required stronger bones, muscles,
hearts, and ligaments. These animals evolved from small ancestors,
gradually developing bodies that could deal with gravitational
Exoskeletons and Endoskeletons
On earth we see two
types of skeletons: exoskeletons (think of exit (outside)
to remember which is which) and endoskeletons, in which the body
is supported by bones inside the skin and muscles.
Exoskeletons are outside
the body and encase it like armor: a good example is a crab in
its shell. Exoskeletons protect the body. They are light and
very strong, and provide attachment places for the muscles inside.
They protect the body from dehydration, predators, and excessive
lead to some difficulties, too. The shell does not grow, so it
has to be shed periodically. It is a struggle to get out of an
exoskeleton, and the animal is vulnerable while the soft new
shell hardens, which may take several hours. Exoskeletons are
also difficult to repair if they have been damaged, although
crabs and many insects can regrow a leg if they lose one. Exoskeletons
are used by crabs, lobsters, insects and spiders. These animals
are bones inside the body. They support the body so that it can
stand and walk, and some structures, like the skull and rib cage,
protect delicate and important organs. Endoskeleton bones provide
a storehouse of calcium for the body, and the calcium can be
drawn on and used in other ways if necessary. Endoskeleton bones
do heal if they get broken, though a doctor's help is often needed
to ensure that they fuse together properly.
Animals with endoskeletons can
grow easily because there are no rigid outside boundaries to
their bodies. They are vulnerable to wounding from the outside,
but repair of the living tissue is usually not a problem. They
have nerve endings in their skins that tell them about the environment:
whether it is hot or cold, wet or dry, rough or smooth. They
feel pain when they are wounded. This information is useful.
Some bodily tissues are supported
by cartilage. This is stiff, rubbery tissue. You have cartilage
in your ears and the tip of your nose. Some sea animals, such
as the shark, have cartilage instead of bone.
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