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Chapter 25
Introduction to Animals
Section 1
What is an Animal?
Characteristics of Animals
• Their cells lack CELL WALLS.
Types of Animals
• INVERTEBRATES do not have a backbone, or VERTEBRAL
• More than 95 percent of all animal species are invertebrates.
• Invertebrates are classified in at least 33 phyla, the largest
taxonomic groups of animals.
• Examples of invertebrates are SEA STARS, JELLYFISHES,
Types of Animals
• CHORDATES exhibit four characteristics during some stage of
development: a dorsal, hollow NERVE CORD; and
NOTOCHORD; and TAIL that extends beyond the anus; and
Types of Animals
• A NOTOCHORD is a supporting rod that runs through the
body just below the NERVE CORD.
• PHARYNGEAL POUCHES are paired structures in the throat
• Most chordates develop a backbone, or vertebral column. They
What Animals Do to Survive
• Animals must maintain HOMEOSTASIS in order to survive.
• One important way to maintain homeostasis is FEEDBACK
INHIBITION, also called negative feedback, in which the
product or result of one process limits the process itself.
What Animals Do to Survive
• In order to maintain homeostasis, animals must:
• GATHER and respond to information;
• OBTAIN and distribute OXYGEN and NUTRIENTS;
• COLLECT and eliminate carbon dioxide and collect wastes.
• They also REPRODUCE.
Section 2
Animal Body Plans and Evolution
Features of Body Plans
• Each animal phylum has a unique organization of body structures
called its “BODY PLAN”. The features of a body plan include
• Levels of organization: CELLS, TISSUES, ORGANS,
Features of Body Plans
• Body symmetry
• RADIAL symmetry: body parts extend from CENTRAL
• BILATERAL symmetry: left and right sides are MIRROR
IMAGES, with front and back ends
Features of Body Plans
• Differentiation of germ layers:
• ENDODERM, the innermost layer
• MESODERM, the middle layer
• ECTODERM, the outermost layer
Features of Body Plans
• Formation of cavity, or FLUID-FILLED space between the
digestive tract and the body wall:
• A true COELOM (found in most complex animal phyla)
develops in the MESODERM and is lined with tissue derived
from the MESODERM
Features of Body Plans
• A PSEUDOCOELOM is only partially lined with MESODERM
Features of Body Plans
• Some invertebrates lack a body cavity and some only have a
primitive, JELLYLIKE layer between ECTODERM and
Features of Body Plans
• Patterns of embryological development
• Sexual reproducing animals begin life as a ZYGOTE, or
fertilized egg
• The zygote develops into a hollow ball of cells, the
Features of Body Plans
• The blastula folds in on itself and creates a tube that becomes the
DIGESTIVE TRACT; the tube has a single opening, the
• In PROTOSTOMES (most invertebrates), the blastopore
becomes the mouth
• In DEUTEROSTOMES (chordates and echinoderms), the
blastopore becomes the anus.
Features of Body Plans
• SEGMENTATION: repeated parts, such as the segments of
• CEPHALIZATION: the concentration of sense organs and
nerves near the anterior (HEAD) end
• LIMB formation: external appendages such as LEGS, FLIPPERS,
The Cladogram of Animals
• The features of body plants provide the evidence needed to build
a CLADOGRAM, or phylogenetic tree, of all animals. Animal
phyla are usually defined by their adult body plans and patterns of
embryological development.
The Cladogram of Animals
• The characteristics of animals VARY within each phylum.
• Each phylum may be thought of as an “EVOLUTIONARY
EXPERIMENT.” Phyla with successful body plans have survived.