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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. Are limiting
determinant of future adaptational variants.
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 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: Three body plans are
possible: Acoelomate (no body
cavity) Pseudocoelomate (body
cavity between endoderm & mesoderm)
Coelomate (body cavity surrounded by 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.
9-19
Components of Metazoan Bodies
Extracellular Components - noncellular components of metazoan animals:Body fluidsExtracellular structural elements
Tissue Structure and Function
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
Cellular Components - Tissues
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.