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. Copyright © McGraw-Hill Education Animal Body Plans

Development of Tissues The first major change in body plan was the development of tissues. Sponges are the only animal without true tissues. Copyright © McGraw-Hill Education Animal Body Plans

Symmetry Symmetry is the next branching point after tissues. Symmetry is the balance or similarity of body structures of an organism. Copyright © McGraw-Hill Education Animal Body Plans

Symmetry Asymmetry Radial symmetry Bilateral symmetry 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. Copyright © McGraw-Hill Education Animal Body Plans

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. Copyright © McGraw-Hill Education Animal Body Plans

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. Copyright © McGraw-Hill Education Animal Body Plans

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. Copyright © McGraw-Hill Education Animal Body Plans

Body Cavities Pseudocoelomates A pseudocoelom is a fluid-filled body cavity that develops between the mesoderm and the endoderm. Only partially lined with mesoderm. Copyright © McGraw-Hill Education Animal Body Plans

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 Copyright © McGraw-Hill Education Animal Body Plans

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. Copyright © McGraw-Hill Education Animal Body Plans

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. Copyright © McGraw-Hill Education Animal Body Plans

Segmentation Segmented animals can be “put together” from a succession of similar parts. Can survive damage to one segment Movement is more effective Copyright © McGraw-Hill Education Animal Body Plans