There have been five key transitions that have occurred in the evolution of the animal body plan.

1. Evolution of tissues

          Porifera including sponges have no tissues.

          All other phyla of animals have some tissues with specialized cells to perform specific functions.

2. Evolution of symmetry

          Radial symmetry parts arranged around a central axis in any plane are mirror images

                                  - includes Cnidaria (jellyfish, hydra, coral, sea anemones) and Ctenophora

          Bilateral symmetry parts are arranged around a central axis that divides the body into right/ left halves

- includes all other phyla

3. Evolution of a body cavity

          Presence of a space within the body allows for more development of internal organs to perform specific functions, such as digestion, transport, and reproduction

                  Acoelomates have no cavity or coelom and include Platyhelminthes

                    Pseudocoelomates have a cavity that develops between the endoderm and mesoderm of the embryo and include Nematodes and Rotifers

                    Coelomates have a cavity that develops within the mesoderm of the embryo.  This means that there is contact between the endoderm and mesoderm during embryonic development and that contact induces the development of more complex structures in the organ systems.

4.  Evolution of protostome and deuterostome development

          In protostome development, the blastopore becomes the mouth, and no single embryonic cell can develop into an entire organism.

          In deuterostome development, the blastopore becomes the anus, and any embryonic cell can develop into an entire organism.

 

 

 

 

5.  Evolution of segmentation

          Segments or repeating parts of the body may provide 2 advantages

1.     if one segment is damaged, other segments may allow survival

2.     locomotion is more effective because areas of the body can be moved independently