Kingdom Animalia for AP Biology

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By — McGraw-Hill Professional
Updated on Oct 24, 2011

Practice problems for these concepts can be found at: Taxonomy and Classification Review Questions for AP Biology


Animals are the final kingdom to be discussed in this chapter. There are some characteristics that separate animals from other organisms:

As is the case with all of the other kingdoms in this chapter, you do not need to become the master of animal phylogeny and taxonomy. But it is definitely useful to know the general evolutionary history of the animal kingdom and how it diversified so quickly over time (Figure 13.1).

Many people believe that the original common ancestor that started the whole process of animal evolution was most likely the choanoflagellate. During the evolutionary progression from choanoflagellate to present date, there are four major branchpoints on which you should focus. Let's take a look at all the important changes that have allowed such diversity of life.

Kingdom Animalia

The first major branchpoint occurred after the development of multicellularity from choanoflagellates. Off this branch of the tree emerged two divisions:

  1. Parazoa: sponges; these organisms have no true tissues.
  2. Eumetazoa: all the other animals with true tissue.

After this split into parazoa and eumetazoa, the second major branchpoint in animal evolutionary history occurred: the subdivision of eumetazoa into two further branches on the basis of body symmetry. The eumetezoans were subdivided into

  1. Radiata: those that have radial symmetry, which means that they have a single orientation. This can be a top, a bottom, or a front and back. This branch includes jellyfish, corals, and hydras.
  2. Bilateria: those that have bilateral symmetry, which means that they have a top and a bottom (dorsal/ventral) as well as a head and a tail (anterior/posterior).

The next major split in the phylogenetic tree for animal development involved the split of bilateral organisms into two further branches—one of which subdivides into two smaller branches:

  1. Acoeleomates: animals with no blood vascular system and lacking a cavity between the gut and outer body wall. An example of a member of this group is the flatworm.
  2. Animals with a vascular system and a body cavity.
    • Pseudocoelomates: animals that have a fluid-filled body cavity that is not enclosed by mesoderm. Roundworms are a member of this branch.
    • Coelomates: a coelom is a fluid-filled body cavity found between the body wall and gut that has a lining. It comes from the mesoderm.

The final major branchpoint comes off from the coelomates. It branches into two more divisions:

  1. Protostomes: a bilateral animal whose first embryonic indentation eventually develops into a mouth. Prominent members of this society include annelids, arthropods, and mullusks.
  2. Deuterostomes: a branch that includes chordates and echinoderms. The first indentation for their embryos eventually develops into the anus.

These two divisions differ in their embryonic developmental stages. As already mentioned, the protostomes' first embryonic indent develops into the mouth, whereas for the dueterostome, it becomes the anus. Another difference is the angle of the cleavages that occur during the early cleavage division of the embryo. A third difference is the tissue from which the coelom divides.

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