Themes > Science > Zoological Sciences > Animal classification > Polygenetic Tree of Kingdom Animalia > Evolutionary History of Animal Groups

Coelomates are animals that have internal body cavities, or coeloms. Humans are coelomates, since we have an abdomenal cavity containing digestive organs, some of the excretory and reproductive organs, and a thoracic cavity that contains the heart and lungs. Coelomates also form a variety of internal and external skeletons. External skeletons and coeloms appeared during the Cambrian-Ordovician time. These skeletons offered several advantages to their producers:
  1. Secretion of a mineral shell that allowed the animal to use the shell as a mineral repository.
  2. Protection from drying out in the intertidal zone during low tides.
  3. Protection from predators.
  4. Sites for anchoring muscle attachments, offering new patterns of locomotion and increased strength.























First appearances and relative diversity (width of shaded area) for major groups of animals. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.


by M.J. Farabee
Information provided by: http://www.emc.maricopa.edu