600-555 million years ago



chordate cross section

From simpler ancestral worms, coelomate worms evolved which possessed a body cavity (a coelom) in which their organs were suspended.

All higher animals (including ourselves) are referred to as coelomates because they possess a coelom. A coelom is an internal body cavity in which a number of organs are hung (in humans the coelom contains the heart, lungs, stomach, liver, and the intestines). In the above cross section of a primitive chordate, note that the gastrointestinal tract (GI tract) and gonads form in a cavity unlike the somites (blocks of muscle) or the notochord. Flatworms lack a coelom and other worms are referred to as "pseudo-coelomates". The most advanced worms are true coleomates as are all groups of higher animals which evolved from worms (such as mollusks, arthropods, and vertebrates).

There are two groups of coelomate animals: protostomes and deuterostomes. Although protostomes compose the majority of animals in both fossil and modern communities, most of this course will be spent studying the deuterostomes, especially the vertebrates.