Radial and bilateral symmetry, cephalization, and the presence of a coelom are important characteristics in invertebrate evolution. Radial symmetry is seen in organisms like sea anemones where body parts repeat around the center, while bilateral symmetry is a single dividing line making two halves in worms and arthropods. Cephalization concentrated sense organs and nerves at the front, allowing more sophisticated responses. A true coelom is a fluid-filled cavity lined with mesoderm tissue, found in more complex phyla.

1. 29-1 Invertebrate Evolution

2. Radial SymmetryBody plan in which body parts repeat around the center of the body; characteristic of sea anemones and sea stars.

3. Bilateral SymmetryBody plan in which only a single, imaginary line can divide the body into two equal halves; characteristic of worms, arthropods, and chordates.

4. cephalizationConcentration of sense organs and nerve cells at the front of an animal’s body.

5. CoelomFluid-filled body cavity lined with mesoderm

6. Key ConceptAll invertebrates except sponges exhibit some type of body symmetry.

7. Key ConceptInvertebrates with cephalization can respond to the environment in more sophisticated ways than can simpler invertebrates.

8. Key ConceptMost complex animal phyla have a true coelom that is lined completely with tissue derived from mesoderm.