Themes > Science > Zoological Sciences > Animal classification > Polygenetic Tree of Kingdom Animalia > Protostome and Deuterostome Conditions > Protostome and Deuterostome Conditions

Protostomes (mollusks, annelids, and arthropods) develop so that the first opening in the embryo is the mouth (protostome = first mouth). Protostomes are bilaterally symmetrical, have three germ layers, the organ level of organization, the tube-within-a-tube body plan, and a true coelom. The coelom, a body cavity between the digestive tract and body wall completely lined by mesoderm allows the digestive system and body wall to move independently. Because of this, internal organs can be more complex. Coelomic fluid assists respiration and circulation by diffusing nutrients, and excretion by accumulating wastes. This fluid functions in place of several organ systems in the higher animals. The coelom may serve as a storage area for eggs and sperm, facilitating development of these gametes within the animal body. Coelomic fluid protects internal organs and also serves as a hydrostatic skeleton. Protostomes develop their embryo by spiral cleavage.

Deuterostomes (as exemplified by the echinoderms and chordates) develop the anus first, then the mouth at the other end of the embryo. Deuterostomes are coelomate animals these embryological characteristics:

  • Radial cleavage in embryonic cell division: the daughter cells sit on top of previous cells.
  • Fate of cells is indeterminate; if embryonic cells are separated, each one develops into a complete organism.
  • The blastopore is associated with the anus, and the second embryonic opening is associated with the mouth.


















































Differences in cleavage between the embryos of protostomes and deuterostomes. Images 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.

The previous chapter looked at the protostomes (annelids, molluscs, and arthropods). This chapter will examine the deuterostomes, primarily the echinoderms and chordates.


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