Animal Diversity

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Animal Diversity

• Text pages 634-642

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Two Views of Animal Diversity

• The traditional phylogenic tree of animals is
  based mainly on grades in body plans, the
  anatomic features and embryonic development.
• However, since then many discoveries have been
  made about molecular make-up and its challenged
  the theories that formulated the traditional
  tree, therefore, molecular systematists are
  moving some branches around on the phylogenetic
  tree of animals.

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The Traditional Trees Basis

• The traditional phylogenetic tree is based on the
  anatomical aspect of the animals called a body
  plan, as well as their embryonic development.
• The branches on the tree are called grades. These
  grades define the body plan features shared by
  the animals.
• There are four main dichotomies that divide the
  diversity of the body plans. They are as follows

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This is the first of two views, the traditional
tree, based on anatomical structures and
embryonic development, as the following explains.
Traditional Phylogenic Tree
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The Parazoa-Eumetazoa Dichotomy The organisms
that lack true tissue, are placed in the
parazoans (means beside the animals) category.
Sponges are an example of parazoans. Tissue is
basic among most animals, and they fit into the
eumatazoans.
The Radiata- Bilateria Dichotomy The eumetazoans
are separated into two different groups. The
groups are based partly on symmetry. There is
radial symmetry, and two-sided symmetry.
Radial symmetry is when the anatomy of an
organism can be divided into more than two parts.
Two-sided symmetry, also called bilateral
symmetry is when an organism can only be divided
in half. This would be due to a head, or a tail,
for example. They have a dorsal (top) side and a
ventral (bottom) side, as well as an anterior
(head) and posterior (tail). Bilateria generally
have cephalization, this is an evolutionary trend
in which there is sensory equipment in the
anterior end, also includes the development of a
central nervous system, primarily in the head,
but extending to the tail via a nerve cord.
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The Acoelomate, Pseudocoelomate, Coelomate
Grades The bilateria category is divided into
these groups acoelomate, pseudocoelomate, and
coelomate.
Acoelomates They have solid bodies, meaning that
there is no cavity between the digestive tract
and outer body wall. Generally have
tube-within-a-tube body structure.
Pseudocoelmates Many animals have body cavities,
fluid filled space between the digestive tract
and the outer body wall. There are differences in
how they develop. The cavity is not completely
lined by tissue derived from mesoderm.
Coelomates They have a true coelom, the fluid
filled body cavity is completely lined in tissue
derived from the mesoderm. The inner and outer
layers are connected dorsally and ventrally to
form mesenteries, which suspend the internal
organs.
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The Protostome- Deuterostome Dichotomy The
Coelomates are divided into two grades,
protostomia and deterostomia. They are
distinguished by differences in the way they
develop.
Deuterstomes Cleavage The zygotes undergo
radial cleavage, in the eight-cell stage, the top
four sit directly on top of the underlying four.
They are also categorized by indeterminate
cleavage, its unpredictable as to how the embryo
will develop. Coelom formation Develops via
enterocoelous the mesoderm buds from the wall of
the archenteron and hollows to become the
coelomic cavities. Blastopore fate The anus is
formed by the blastopore the mouth is the second
opening.
Protostomes Cleavage The zygotes undergo spiral
cleavage, an embyronic development in which each
tier of cells sit in the grooves of the
underlying tier. This is a determinate cleavage,
it defines how the embryo will look, even at the
eight-cell stage. Coelom Formation In
gastrulation, the developing of the digestive
tube, there are differences. As the archenteron
develops, the initially solid masses of mesoderm
splits to form a the coelomic cavities this is
called schizocoelous. Blastopore Fate A
blastopore is the opening of the archenteron, it
is generally the mouth or the anus. For
protostomes, the mouth is formed by the
blastopore.
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This view is based on nucleotide sequences in
ribosomal RNA.
Molecular View
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Comparison
Similarities between the trees -the deepest
branches parazoa- eumetazoa, and radiata-
bilateria -the deuterostome clade Differences
between the trees -the two main protostome
clades molecular evidence shows two groups
within the clade lophotrochozoa and ecdysozoa.
-the acoelomates and pseudocoelomates are in
another clade
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Molecular v. Anatomical