Architectural Pattern of
an Animal
Chapter 9
The Appearance of Major Body
Plans
 The Cambrian
explosion marks the
earliest fossil
appearance of all
major groups of living
animals plus some
groups that are only
known from fossils.
 All major body plans
appeared at this time.
 Result of extensive
selection.
Body Plans
 One way in which zoologists categorize the diversity of
animals is according to general features of morphology
and development.
 A group of animal species that share the same level of
organizational complexity is known as a grade.
Hierarchical Organization – 5 Levels
 Protoplasmic grade of organization
– protists are the simplest eukaryotes,
but they still carry out life functions and
show division of labor among the
various cell structures.
 Metazoans are multicellular animals
that have cells specialized for
particular functions. This is the
Cellular grade of organization.
 Shown by the simplest metazoans –
Volvox, sponges.
Hierarchical Organization – 5 Levels
 Cell-tissue Grade – Usually,
specialized cells are grouped
together and perform their
common function as a coordinated
unit, a tissue.
 Jellyfish
 Tissue-organ Grade – Tissues
are then assembled into organs
like the heart (primarily muscle
tissue, but connective, nervous,
and epithelial also present).
 Flatworms
Hierarchical Organization – 5 Levels
 Organ-system grade – In the highest level of
organization, organs work together as organ systems
like the circulatory system.
Animal Body Plans
 Body plans are constrained by ancestry, major features
may become modified, but are rarely lost.
 Animal body plans differ in their grade of organization,
body symmetry, number of germ layers, and type of
body cavity.
Symmetry
 Spherical symmetry
occurs when any
plane passing
through the center
divides the body into
mirror image halves.
 Mostly found among
the protists.
Symmetry
 Radial symmetry applies
when more than two planes
passing through the
longitudinal axis can divide
the organism into mirror
image halves.
 Jellyfish
 Biradial symmetry – two
planes will divide the
organism.
 Comb jellies
Radiata
 The Cnidarians (jellyfish,
corals & sea anemones)
and Ctenophores (comb
jellies), the radial or
biradial animals,
comprise the Radiata.
 No front/back
 Weak swimmers
 Can interact with
environment in all
directions.
Symmetry
 Bilateral symmetry
is found in organisms
where one plane can
pass through the
organism dividing it
into right and left
halves.
 Better for directional
movement.
 Monophyletic group
called Bilateria.
Cephalization
 Bilateral symmetry is associated with cephalization,
differentiation of a head.
 Nervous tissue, sense organs, and often the mouth are
located in the head.
 Advantages for organisms moving head first – directional
movement.
 Elongation along anteroposterior axis.
Regions of a Bilaterally Symmetrical
Animal
 Anterior-posterior (transverse plane)
 Dorsal-ventral (frontal plane)
 Left-right (sagittal plane)
 Proximal-distal
 Medial-lateral
Body Cavities
 Examples of body cavities include the gut, blastocoel,
and a pseudocoel or coelom.
 The gut forms from the archenteron during gastrulation.
 The blastocoel persists in some, but usually fills with
mesoderm.
 Pseudocoel and coelom are fluid filled body cavities that
cushion organs and provide support.
Body Cavities
 In protostomes, mesoderm forms
as endodermal cells from near
the blastopore migrate into the
blastocoel.
 Three body plans possible:
 Acoelomate (no body
cavity)
 Pseudocoelomate (body
cavity between endoderm &
mesoderm)
 Coelomate (body cavity
surrounded by mesoderm)
Body Cavities
 Coeloms surrounded by
mesoderm can arise in two
ways:
 Schizocoely – mesodermal
cells fill the blastocoel,
forming a solid band of tissue
around the gut, then a space
opens inside the mesodermal
band.
 Enterocoely – portions of the
gut lining form pockets that
pinch off and form a ring of
mesoderm.
Developmental Patterns
 Sponges develop only to
blastula stage, then
reorganize to form adult.
 Gastrulation allows
animals to proceed to
tissue level organization.
 Diploblastic – 2 germ
layers
 Cnidarians,
Ctenophores
 Triploblastic – 3 germ
layers
Other Key Features of Body Plans
 In some organisms, the gut does not form a complete
tube.
 Waste must come back out the mouth.
 Food must be digested & waste expelled before eating
again.
 A complete gut forms a tube within a tube body plan.
Other Key Features of Body Plans
 Segmentation is a serial
repetition of similar body
segments along the body.
 Each segment is a
metamere or somite.
 May include external &
internal components.
 Obscured in many animals,
like humans.
 Permits greater body
mobility and complexity of
structure & function.
Components of Metazoan Bodies
 Extracellular Components - noncellular components of
metazoan animals:
 Body fluids
 Extracellular structural elements
Tissue Structure and Function
 Cellular Components Tissues
 A tissue is a group of
similar cells specialized
for performing a common
function.
 Different types of tissues
have different structures
that are suited to their
functions.
 Tissues are classified
into four main categories:
 Epithelial
 Connective
 Muscle
 Nervous
Epithelial Tissue
 Epithelial tissue
covers the outside of
the body and lines
organs and cavities
within the body.
 Squamous, cuboidal,
columnar.
 Simple vs. stratified.
Connective Tissue
 Connective tissue functions
mainly to bind and support other
tissues.
 Contains sparsely packed cells
scattered throughout an
extracellular matrix.
Muscle Tissue
 Muscle tissue is composed of long cells
called muscle fibers capable of contracting in
response to nerve signals.
 Smooth
 Skeletal
 Cardiac
Nervous Tissue
 Nervous tissue senses
stimuli and transmits
signals throughout the
animal.
 A neuron (nerve cell)
receive signals at the
dendrites and send
them out via the axons.
Complexity and Body Size
 Increased complexity
allows for an increase in
body size.
 Larger size decreases the
surface area to volume
ratio.
 Necessitates complex
systems for respiration,
nutrition, and excretion –
diffusion not adequate.
 Buffers environmental
fluctuation.
 Escape predators.
Complexity and Body Size
 Cost of maintaining
body temperature is
less per gram of
body weight than in
small animals.
 Energy costs of
moving a gram of
body weight over a
given distance less
for larger animals.