Download Chapter One- Introduction

Chapter One- Introduction
About this Book:
Welcome to the exotic world of the animal kingdom. When most people think of an animal,
they quickly name a vertebrate anaimal but there are so many more types of animals out there. This
book is designed to introduce you to some of the more unusual and lesser-known types of animals.
You'll meet some odd creatures along the way and hopefully gain an appreciation for animals that you
might otherwise just want to squash when you see them.
What is an Animal?
Okay, this sounds like a dumb question but it isn't. Not all animals have what we would
consider to be “normal” animal characteristics like two eyes or four legs. An animal must have, at
minimum, most of the following four characteristics to belong to the animal kingdom. First, it must be
multicellular with no cell walls. Yes, there are some animal-like single-celled organisms out there but
they have their own kingdom (the protists). Second, all animals are heterotrophs. This means that
animals must consume other living things to get energy. They cannot get energy to run their bodies
from the sun like a plant. Third, most animals reproduce sexually to form a zygote and then those
cells differentiate, or form new specialized cells that will go on to become different body parts like the
heart, lungs or brain. Some animals can also reproduce asexually but it isn't very common. And lastly,
most animals can move in their environment. A few, like barnacles, get glued in place but since they
fit the other three characteristics, they are still considered to be animals.
Vertebrates and Invertebrates:
The animals you are most familiar with are vertebrates, animals with a backbone. There are
five main classes of these: fish, reptiles, amphibians, birds and mammals. However, invertebrates,
animals that lack a backbone, are far more numerous than all of the vertebrates put together. They
include the simplest animals, the sponges, to far more complex and intelligent creatures like the
octopus.
Body Symmetry:
One of the most obvious things you notice about an animal when you see it is its body shape.
The body plan can range from no shape (no symmetry) for simple animals like sponges to bilateral
symmetry for more complex animals. No symmetry means that the animal has no specific form and is
kind of a blob. There is no head end and no brain. Animals that are radially symmetrical have a top
and a bottom but there is no difference between a front and back side. You could hypothetically cut
them anywhere down the middle and get two equal sides. Most animals are a bit more complex than
that and have bilateral symmetry. This means they have a top (aka anterior – head end), bottom
(aka posterior – rear end), front side (aka dorsal) and a back side (aka ventral). More importantly.
This means that they must have some form of a head and therefore, some form of a brain even if it is
very small. So, in general, insects must be a lot smarter than sponges and sea anemones.
Basic Terminology:
If we are going to be spending a lot of time discussing animals, we need to explain what a few
basic terms mean. Some animals are segmented. This means that they have a series of repeating
similar units that are attached to each other like an earthworm.
Segments in an earthworm
Some animals wear their skeletons on the outside kind of like armor. These are exoskeletons
and are most commonly found in the arthropods which includes the insects and crustaceans.
Unfortunately, in order for an animal with an exoskeleton to grow, they must shed their outer covering
and grow a new one. This leaves them vulnerable for a short period of time. It also makes for a tasty
meal when you eat soft-shell crabs! Animals with their skeletons tucked inside have endoskeletons.
The endoskeleton acts as a support and is surrounded with soft tissue.
Exoskeleton of a cicada
Most animals release carbon dioxide as a waste gas and require oxygen to live. They need a
method to bring oxygen to body cells and remove waste CO2 from their cells. The simplest creatures
like sponges and anemones live in water and use water to move CO2 and O2 directly in and out of their
bodies. Slightly more complex critters like arthropods and mollusks have an open circulatory system.
Blood flows from their heart through vessels into the main body cavity but then just flows loose in the
body. This is not effective in larger, more complex animals so they have a closed circulatory system.
In a closed system, the blood moves through the body only in vessels and never just oozes everywhere.
All animals must eat and thank goodness most animals have a gut. A gut is a one way track
starting at the mouth and ending at the anus. This means that food flows through the animal, is
digested and then excreted out the other end. A few unlucky animals only have one opening so their
wast exits through the same central chamber. Ugghhhh!
No mouth, no gut and now
you know what that means!
Gross....
Most animals have some sort of anterior or head end. We know it is the head end because it
tends to react to the environment and move using one side. This “head” end is the result of
cephalization, or concentration of sensory and nerve tissues in one spot. For more complex creatures,
these sensory and nerve tissues form a brain that helps the animal make sense of its environment. (You
might not agree that all creatures with brains make sense of their environment though. I know quite a
few senseless people!)
Some animals are born and develop directly into adults. Oh sure, they grow and change a little.
They get bigger or change color or develop horns but essentially the babies are very recognizable
compared to the adults. This is direct development and is the most common form of development for
most vertebrates except fish and some amphibians. Many invertebrates, on the other hand, go through
a larval stage that looks very different from the adult. This is called indirect development. Larva can
live by themselves in the environment and often have very different habitats than the adult.
So who looks more like
the parent? The kitten or
the caterpillar. Now you
can see why it's called
direct development.
Photo Credits:
Symmetry diagram: http://evolution.berkeley.edu/evolibrary/article/0_0_0/arthropods_04
Earthworm: http://www.biologycorner.com/worksheets/earthworm.htm
human endoskeleton:
http://www.robotstoreuk.com/ROBOT%20PAGES/Pages%20IMAGES/The%20Human%20EndoSkel
eton.jpg
Cicada exoskeleton: http://cccmkc.edu.hk/~keikph/Kadoorie%20farm/Exoskeleton%20of%20cicada_larger%20picture.htm
Sea anemone: http://earthguide.ucsd.edu/hughes2001/acct/dgomez/anemone/anemone.htm
Butterfly life cycle: http://www.naturebase.net/plants_animals/butterflies/images/butterfly_lifecycle.jpg
Cat: http://www.edmondpetclinic.com/
Kitten: http://www.blogography.com/archives/2004/12/kitten.html