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.

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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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