HOM (1)

M A I N
C H A R C T E R I S T I C S
O F A N I M A L S
There are four main characteristics that are
used to classify animals: symmetry, body
cavity, tissue, and vertebral column.
The first
characteristic is
symmetry. Animals
generally fall into
two categories - they
either have radial or
bilateral
symmetry. Organis
ms like sea stars
have radial
symmetry, while
organisms such as
humans have
bilateral symmetry
The next thing is
body tissue. The
more complex an
organism is, the
more distinctive
layers of tissue it has.
There are three
main layers:
Ectoderm - the
outermost layer
Endoderm - the
innermost layer
Mesoderm - the
middle layer
As for vertebral
column, there are
two key groups:
invertebrates and
vertebrates. Inverteb
rates are animals
without a backbone,
while vertebrates are
animals with a
backbone. As we
look at the evolution
of animals, we will
first look at the more
simple invertebrates
and then consider
the more complex
vertebrates.