Animal Architecture
 Levels
of organization in organismal
complexity.
 There
are 5 major grades of organization
each being more complex than the
previous.
Levels of organization

1. Protoplasmic level: occurs in unicellular
organisms. Organelles within the cell carry out
specialized functions. Protozoans are examples.

2. Cellular level: Cells are aggregated and cells
engage in a division of labor, being specialized
for particular tasks. Colonial protozoan groups
with distinct somatic and reproductive cells and
sponges are examples. (Animals that are
multicellular are referred to as Metazoans).
Levels of organization
 3.
Cell-tissue level: Similar cells
aggregate into patterns or layers forming
tissues. Nerve net in Cnidarians (e.g.
jellyfish) is example of a tissue.
Levels of organization
 4.
Tissue-organ level: Organs are made up
of more than one kind of tissue and have a
specialized function. Flatworms
(Platyhelminthes) generally represent this
level having organs such as eyespots and
reproductive organs, but their reproductive
organs are organized into level 5 an organ
system.
Flatworm (Turbellaria)
Levels of organization
 5.
Organ-system level: Organs work
together to perform functions. Most
complex level of organization.
 Examples
of organ systems include
circulatory, reproductive, digestive,
respiratory. Most animal phyla exhibit this
level of organization.
Animal symmetry
 There



are three types of symmetry.
Spherical
Radial
Bilateral
Animal symmetry

Spherical symmetry occurs mainly among
protozoans.
 Radial symmetry occurs among the Cnidarians
(jellyfish) and Echinoderms (starfish, sea
urchins).
 Bilateral symmetry commonest form of
symmetry. Strongly associated with
cephalization or development of a head with
associated sensory and feeding apparatus.
A variety of descriptive terms are used to
describe orientation in bilateral animals.
Development of body plans
 An
animal’s body results from division of
cells during embryonic development.
 Differences
in developmental patterns
have been used to classify more complex
animals so an understanding of basic
embryology is necessary to follow this.
Process of development

Once an egg is fertilized it becomes a zygote.
This cell divides into a large number of cells
called blastomeres.

Cleavage of cells proceeds until a fluid-filled
hollow ball of cells is formed. This is a blastula.

In multicellular animals other than sponges the
blastula invaginates to begin forming the future
gut. At this stage the embryo is a gastrula.
Process of development

The invaginating layer of cells, which will give
rise to the gut, form a germ layer called the
endoderm. The endoderm surrounds and
defines a body cavity called the gastrocoel.

The cells not involved in forming the invagination
constitute another germ layer the ectoderm. The
ectoderm surrounds a cavity called the
blastocoel.
gastrocoel
Process of development
 When
the invaginating gastrocoel forms a
complete tube by forming a second
opening to the outside it is then called the
gut.
 In
the cnidarians (jellyfish, sea anemones)
no second opening develops.
Process of development
 In
most animals (but not cnidarians, which
are two-layered or diploblastic) a third
germ layer of cells called the mesoderm
develops.
 The
mesoderm gives rise to many internal
organs. Organisms with mesoderm are
called triploblastic having three germ
layers.
Germ layers

Endoderm: innermost germ layer of an embryo.
Forms the gut, liver, pancreas.

Ectoderm: Outer layer of cells in early embryo.
Surrounds the blastocoel. Forms outer
epithelium of body and nervous system.

Mesoderm: Third germ layer formed in gastrula
between ectoderm and endoderm. Gives rise to
connective tissue, muscle, urogenital and
vascular systems and peritoneum.
Process of development
 The
way in which the mesoderm forms,
and whether or not a cavity (called a
coelom) develops within it, are important
characters in deciphering the relatedness
of animal groups.
Coeloms
 The
coelom is a cavity entirely surrounded
by mesoderm.
 A coelom provides a tube-within-a-tube
arrangement which has many advantages:



Allows flexibility in arranging visceral organs
permits greater size and complexity by
exposing more cells to surface exchange
fluid-filled ceolom can act as a hydrostatic
skeleton
Coeloms

Triploblastic organisms (organisms with three
germ layers including mesoderm fall into one of
three different coelomic states:



Acoelomate: mesoderm fills the blastoceol, no cavity
occurs in the mesoderm. Flatworms and nemerteans.
Pseudocoelomate: mesoderm lines only outer edge
of blastocoel. No peritoneal lining develops.
Nematodes and rotifers.
Eucoelomate: Have a true coelom derived from
mesoderm and lined with peritoneum. Arthropods,
annelids, mollusks, echinoderms, vertebrates.
Both eucolomate
Protostomes and Deuterostomes
 Within
the eucolomates there are two
major evolutionary lineages that split early
in the history of animals and follow quite
different developmental pathways.
These are the protostomes “mouth first” and
deuterostomes “mouth second”.
Important differences in development
between protostomes and deuterostomes

The differences in development that distinguish
the protostomes and deuterostomes include:




Whether cleavage of cells in the early zygote is spiral
or radial.
Whether or not, if the early blastomere is separated,
each cell can develop into a normal larva or not.
Whether the blastopore ultimately forms the mouth or
anus of the organism.
Whether or not the organism possesses a coelom
and how that coelom is formed.
Figure 08.10
Protostomes and Deuterostomes
 Protostomes
include the annelids,
mollusks, and arthropods.
 Deuterostomes
include the echinoderms
and vertebrates.