Evolution of Animal Body Plans• Anatomical features in animals’ body plans mark the branching
points on the evolutionary tree.
• Relationships on this tree are inferred by studying similarities in embryological development and shared anatomical features.
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Animal Body PlansCopyright © McGraw-Hill Education
Development of Tissues• The first major change in body plan was the development of
tissues.
• Sponges are the only animal without true tissues.
Symmetry• Symmetry is the next branching point after tissues.
• Symmetry is the balance or similarity of body structures of an organism.
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SymmetryAsymmetry• Irregular shape, no symmetry or balance in body structures.
Radial symmetry• An animal with radial symmetry can be divided along any plane,
through a central axis, into roughly equal halves.
Bilateral symmetry• Bilateral symmetry means an animal can be divided into mirror
image halves along only one plane.
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Symmetry
Bilateral symmetry• Animals with bilateral symmetry also have anterior (head) and
posterior (tail) ends.
• This body plan is called cephalization, and involves a tendency to concentrate nervous tissue and sensory organs at the anterior end of the animal.
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Body Cavities• Animals with bilateral symmetry have a gut, which is either a
sac inside the body or a tube that runs through the body, where food is digested.
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Body Cavities
Coelomates• A coelom is a mesoderm-lined, fluid-filled cavity between
the gut and the outside body wall.
• Specialized organ and body systems develop from the mesoderm that encloses and lines the coelom.
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Body Cavities
Pseudocoelomates
• A pseudocoelom is a fluid-filled body cavity that develops between the mesoderm and the endoderm.
• Only partially lined with mesoderm.
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Body Cavities
Acoelomates• Acoelomates do not have a coelom.
• Have solid bodies without a fluid-filled body cavity between the gut and the body wall
• Nutrients and wastes diffuse between cells; no circulatory system
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Development in Coelomate Animals
Protostomes• Organisms that are protostomes develop mouths from the first
opening in the gastrula.
• As the embryo develops, the mesoderm splits down the middle to form the coelom.
Animal Body PlansCopyright © McGraw-Hill Education
Development in Coelomate Animals
Deuterostomes• In organisms that are deuterostomes, the anus develops from the
first opening in the gastrula.
• Coelom develops from two pouches in the mesoderm.
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Segmentation• Segmented animals can be “put together” from a succession of
similar parts.
• Can survive damage to one segment
• Movement is more effective
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