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