Chapter 27
Introduction to Animals
Section 1: Characteristics of Animals
Section 2: Animal Body Systems
Section 1
Characteristics of Animals
Objectives:
•Identify the features that animals have in common.
•Distinguish radial symmetry from bilateral
symmetry.
•Summarize the importance of a body cavity.
•Identify how scientists determine evolutionary
relationships among animals.
Section 1
Characteristics of Animals
General Features of Animals
•Heterotrophy Animals are heterotrophs—that is,
they cannot make their own food.
•Mobility Animals are unique in being able to
perform rapid, complex movements.
•Multicellularity All animals are multicellular.
•Diploidy With few exceptions, animals are diploid.
•Sexual Reproduction Almost all animals reproduce
sexually by producing gametes.
Section 1
Characteristics of Animals
General Features of Animals continued
•Absence of a Cell Wall Among the cells of
multicellular organisms, only animal cells lack rigid
cell walls.
•Blastula Formation In all animals except sponges,
the zygote undergoes cell divisions that form a
hollow ball of cells called a blastula.
•Tissues The cells of all animals except sponges are
organized into structural and functional units called
tissues.
Section 1
Characteristics of Animals
Body Symmetry
•Radial Symmetry Animals with radial symmetry
have body parts arranged around a central axis.
•Bilateral Symmetry Animals with bilateral
symmetry have a distinct right and left half, and most
display cephalization.
Section 1
Characteristics of Animals
Internal Body Cavity
•Types of Body Cavities Animals have one of three
basic body plans: acoelomate, psuedocoelomate, and
coelomate.
Section 1
Characteristics of Animals
Body Segmentation
•Segmentation Segmentation in body structure
underlies the organization of all advanced animals.
Section 1
Characteristics of Animals
Kinds of Animals
•Classification There are about 35 animal phyla,
which contain an extraordinary range of body forms
and body systems. Scientists classify animals using
several different types of data, which include
comparing anatomy and physiology, patterns of
development, and DNA. The animal kingdom is
divided into two groups: vertebrates and
invertebrates.
Section 2
Animal Body Systems
Objectives:
•Summarize the functions of the digestive,
respiratory, circulatory, nervous, skeletal, and
excretory systems.
•Compare a gastrovascular cavity with a one-way
digestive system.
•Differentiate open from closed circulatory systems.
•Distinguish asexual from sexual reproduction.
Section 2
Animal Body Systems
Tissues and Organs
•Digestion Simple animals have a gastrovascular
cavity with only one opening, while more-complex
animals have a one-way gut.
•Respiration Simple animals exchange gases directly
through their skin. More complex aquatic animals use
gills, while terrestrial animals use a variety of
respiratory organs, such as lungs.
•Circulation In an open circulatory system,
circulatory fluid leaves the vessels and enters the
body cavity. In a closed circulatory system, blood
remains in the vessels.
Section 2
Animal Body Systems
Tissues and Organs continued
•Conduction of Nerve Impulses While simple
animals have little coordination among their nerve
cells, complex animals have nerve cords and a brain
with associated sensory structures.
•Support While simple animals have a hydrostatic
skeleton, complex animals have either an external
skeleton, called an exoskeleton, or internal skeleton,
called an endoskeleton.
•Excretion For most animals, eliminating wastes is
linked to maintaining the correct water balance in
their body.
Section 2
Animal Body Systems
Reproductive Strategies
•Asexual Reproduction Asexual reproductive
methods include fragmentation, splitting in two, and
parthenogenesis.
•Sexual Reproduction In sexual reproduction, male
and female gametes combine to form a new
individual.