Taxonomy and Classification ❮ 161
Kingdom Animalia
Animals are the final kingdom to be discussed in this chapter. There are some characteris-
tics 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 gen-
eral 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 the present, there have been fourmajor branchpoints on which you should
focus. Let’s take a look at all the important changes that have allowed such diversity of life.
ADAPTATION DESCRIPTION
Cell wall Animals lack cell walls.
Mode of reproduction Sexual reproduction is the norm (although there are
several animals capable of asexual reproduction).
Dominant life cycle stage The diploid stage is usually the dominant generation
in the life cycle.
Motile Most animals are mobile.
Nutritional class Animals are multicellular heterotrophs.
Storage of energy Animals store carbohydrates as glycogen, not starch as
is seen in plants.
Special embryological events Most animals undergo a process in which specialized
tissue layers (endoderm, mesoderm, ectoderm)
form during a process known asgastrulation.
Nervous and muscle tissue Animals (with the exception of sponges) have specialized
nervous and muscle tissue.
Cellular junctions Animal cells contain tight junctions and gap junctions.
s
Radiata
Eumetazoa Acoelomates Protostomes
Choanoflagellate Bilateria Coelomates
Parazoa Animals with
body cavities
Deuterostome
Pseudocoelomates
Figure 13.1 The animal phylogenetic tree.
KEY IDEA
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