Download Evolution in the Animal Kingdom

Evolution of Systems in the Animal Kingdom
Over the course of studying the five kingdoms of living things found on the planet, we have seen a great
diversity in life styles (the strategies used to live and survive). Our projects focused on the animal
kingdom. Even within this kingdom we saw great differences in how the organisms provided blood
(circulatory system), broke down food particles (digestion) and gathered sensory information (nervous
system) within their cells, as well as great differences in reproduction.
Circulatory system:
A circulatory system is a network that carries blood and materials throughout the body. All animals
except the simplest kinds have some type of circulatory system.
In some invertebrates (animals without a backbone), the circulatory system consists of a simple network
of tubes and hollow spaces. Other invertebrates have pumplike structures that send blood through a
system of blood vessels. In human beings and other vertebrates (animals with a backbone), the circulatory
system consists primarily of a pumping organ—the heart—and a network of blood vessels.
Digestive system:
The way organisms in the animal kingdom provide nutrients to their cells varies greatly. Simple organisms,
like sponges, don’t have specialized organs responsible for digestion and each individual cell absorbs and
breaks down food particles (intracellular digestion). Some invertebrates have a space in their body where
digestion begins (extracellular digestion). In this case, the partially digested food particules are
absorbed by individual cells and then the individual cells complete the digestive process. These primitive
digestive systems may have only one opening where food and waste both enter and leave the body or may
be a one-way tract with a mouth and anus. Mammals and other vertebrates have specialized organs
responsible for digestion, leading to the development of digestive systems.
The digestive system is the group of organs that break down food into smaller particles, or molecules, for
use in the organism’s body. This breakdown makes it possible for the smaller digested particles to pass
through the intestinal wall into the bloodstream. The particles are then distributed to nourish all parts of
the body.
Nervous system:
The nervous system is an internal communications network that enables an animal to adjust to changes in
its environment. Almost all animals, except the simplest kinds, have some type of nervous system.
Invertebrate animals have various abilities when it comes to sensing their environment. Sponges (the
most primitive animal) have no nervous system, and hence no ability to respond to their environment. Most
invertebrates have nervous systems that range from a simple network of nerves (Ex.- hydra and jellyfish)
to a highly organized system of nerve cords and ganglia (a primitive brain) (Ex.- earthworms and squid).
In human beings and other animals with backbones, the nervous system consists of the brain, the spinal
cord, and the nerves. The development of a control center(the brain) and spinal cord meant better
sensory/response times in these groups which allowed them the ability to respond more quickly to changes
in their environment. In the case of humans and some other species, the brain allows for “reason” and
“intelligence”.
Reproduction:
Reproduction is the process by which living things create more of their own kind. All types of living
creatures reproduce, from the tiniest bacteria to the largest plants and animals. Without reproduction, all
forms of life would die out.
There are two general types of reproduction—sexual and asexual. In sexual reproduction, a new organism
is formed by the joining of a gamete (sex cell) from one organism with a gamete from another organism.
Human beings and almost all other animals reproduce sexually. In asexual reproduction, a new organism
develops from parts of, or parts produced by, one organism. Living things that reproduce asexually include
bacteria and other simple organisms. Many organisms can reproduce both sexually and asexually. They
include most plants and fungi and certain simple animals, such as sponges.
Scientists believe that the first living things lived in the sea and reproduced asexually. Sexual
reproduction also evolved (developed gradually) in the sea. After organisms began to live on land, they
evolved increasingly complex methods of sexual reproduction.
Sea animals reproduce sexually by means of external fertilization. In this process, the female releases
eggs into the water. Fertilization occurs after a male releases sperm into the water and the sperm unite
with the eggs. Squid and most fish sexually reproduce in this manner.
About 400 million years ago, some organisms left the sea to inhabit land. The new, dry environment
presented problems for existing methods of reproduction. Organisms that reproduced sexually could not
simply release their gametes near one another on land since the gametes would dry up and die. Thus,
organisms evolved new reproduction methods suitable to land conditions; insects developed eggs with
relatively hard shells that could withstand the dry environment while reptiles, birds & mammals developed
internal fertilization. Birds and reptiles copulate and then the female lays the fertilized eggs in a safe
place. Some species protect their eggs and help nurture their babies, while other species don’t. Mammals
have both internal fertilization and development of the baby and then give birth to live young. In this
case, one (the female) or both parents raise their young.
Questions:
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What is an open circulatory system?
What is a closed circulatory system?
Why is a closed circulatory system considered to be the more advanced system?
What is the advantage of having a digestive system?
What is the advantage of having a one-way digestive tract with two openings (a mouth and an
anus)?
Explain the variety of nervous systems seen in the animal kingdom.
What is the advantage of having a brain and spinal cord?
What is asexual reproduction? Give an example of an animal that reproduces in this way.
What is sexual reproduction? Why is it an advantage?
What is internal fertilization? Why is it an advantage?
What is meant by the term “internal development”? Why is it an advantage?
In terms of the organ systems discussed here, compare the simplicity of a sponge to the
complexity of a mammal.
Taken and revised from:
World Book Online Reference Center. 2005. World Book, Inc. 4 Jan. 2005, http://www.worldbookonline.com