1. Animals exhibit different types of symmetry and levels of complexity, ranging from asymmetrical sponges to bilaterally symmetrical organisms like humans. 2. Key transitions in animal evolution include the development of tissues, body cavities, patterns of embryonic development, and segmentation. 3. Animals use homeostasis, negative feedback loops, and positive feedback to maintain internal stability and regulate processes like glucose levels and childbirth.

1. The AnimalBody: Basic Form &
Function
Chapter 33

2. Figure 33.1
• An arctic fox is a complex animal, well adapted to its environment. It changes coat color with the seasons, and has longer fur
in winter to trap heat. (credit: modification of work by Keith Morehouse, USFWS)

3. • Heterotrophy
• obtain organic molecules by ingesting other organisms
• Multicellularity
• No cell walls
• Active movement
• Neurons and muscle cells
• Diversity of form
• Vary greatly in form, ranging in size from microscopic
organisms to enormous ones
• Collagen
3
General Features of Animals (cont.)

4. General Features of Animals (cont.)
4
• Diversity of habitat
• mostly marine, but also freshwater and terrestrial.
• Sexual reproduction
• Most animals reproduce sexually, but some can also
reproduce asexually.
• Tissues
• Cells of most animals are organized into structural and
functional units called tissues.

5. General Features of Animals (cont.)
5
• Embryonic development
• Zygote first undergoes a series of fast mitotic divisions
called “cleavage” that produces a ball of cells.
zygote morula blastula gastrula
cleavage
gastrulation
cleavage
cleavage cleavage

6. 6
Traditional Classification of Animals
Metazoans (multicellular animals) are traditionally divided
into 36 or so distinct phyla based on shared anatomy and
embryology
Metazoans are divided into two main branches:
• Parazoa (sponges) = Lack symmetry and true tissues
• Eumetazoa = Have symmetry and true tissues
• Diploblastic = Have two germ layers
• Triploblastic = Have three germ layers

7. Evolution of the Animal Body Plan
Five key transitions can be noted in animal evolution
1. Symmetry
2. Tissues
3. Body cavity
4. Patterns of Development
5. Segmentation
7

8. 8
1. Evolution of symmetry
̶ Sponges also lack any definite symmetry (asymmetrical)
̶ Eumetazoa have a symmetry defined along an imaginary axis
drawn through the animal’s body
There are two main types of symmetry:
Radial symmetry
̶ Body parts arranged around central axis
̶ Can be bisected into two equal halves in any 2-D plane
Bilateral symmetry
̶ Body has right and left halves that are mirror images
̶ Only the sagittal plane bisects the animal into two equal halves
Evolution of the Animal Body Plan

9. Figure 33.2
• Animals exhibit different types of body symmetry. The sponge is asymmetrical, the sea anemone has radial symmetry, and
the goat has bilateral symmetry.

10. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
a.
Radial Symmetry
b.
Bilateral Symmetry
Ventral
Dorsal
Frontal plane
Sagittal plane
Anterior
Posterior
Transverse
plane
10
Top
Bottom
Back
Front
Comparison of
radial and
bilateral
symmetry

11. 11
Evolution of the Animal Body Plan
2. Evolution of tissues
̶ Parazoa (Sponges – the simplest animals) lack defined
tissues and organs
• Have the ability to disaggregate and aggregate
their cells
• Cells can convert from one type to another
̶ Eumetazoa (all other animals) have distinct and well-defined
tissues
• Have irreversible differentiation for most cell types

12. 12
Evolution of the Animal Body Plan
3. Evolution of a body cavity
Bilaterians have three germ layers
̶ Outer ectoderm (body coverings and nervous system)
̶ Middle mesoderm (skeleton and muscles)
̶ Inner endoderm (digestive organs and intestines)
Body cavity = Space surrounded by mesoderm tissue that
is formed during development

13. 13
Evolution of the Animal Body Plan
Three basic kinds of body plans
̶ Acoelomates = No body cavity
̶ Pseudocoelomates = Body cavity between mesoderm and
endoderm
• Called the pseudocoelom
̶ Coelomates = Body cavity entirely within the mesoderm
• Called the coelom

14. 14
Three body plans
for bilaterally
symmetrical
animals
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Flatworm
Pseudocoelomate
Roundworm
Pseudocoelom
Coelomate
Annelid
Coelom
Ectodermally
derived tissue
Endodermally
derived tissue
Mesodermally
derived tissue
Digestive
cavity
Ectodermally
derived tissue
Endodermally
derived tissue
Mesodermally
derived tissue
Acoelomate
Endodermally
derived tissue
Ectodermally
derived tissue
Digestive
cavity
Mesodermally
derived tissue
Digestive
cavity

15. 15
Evolution of the Animal Body Plan
̶ Coelomates developed a circulatory system to flow nutrients
and remove wastes
̶ Open circulatory system: blood passes from vessels into sinuses,
mixes with body fluids, and reenters the vessels
̶ Closed circulatory system: blood moves continuously through vessels
that are separated from body fluids

16. 16
Evolution of the Animal Body Plan
4. Evolution of different patterns of development
Mitotic cell divisions of the zygote form a morula and then a
blastula.
Gastrulation produces a two layer gastrula-stage embryo with:
Blastopore = Opening to outside
Archenteron = Primitive digestive chamber

17. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Cleavage
Fate of
Embryonic Cells
Fate of
Blastopore
Formation of
Coelom
Four-cell
embryo
Determinate
development
SpiralianProtostomesDeuterostomes Cell
excised
Development
arrested
Indeterminate
development
Cell
excised
Axis
Side view Top view
Four-cell
embryo
Side view Top view
Radial cleavage
Normal
embryos
Blastopore
becomes mouth
Blastopore
becomes anus
Archenteron
Mesoderm
Mesoderm
Archenteron
Mouth
Coelom
Anus
Axis
Axis
Spiral cleavage
17

18. 18
Evolution of the Animal Body Plan
Bilaterians can be divided into two groups:
̶ Protostomes develop the mouth first from or near the
blastopore
• Anus (if present at all) develops either from blastopore or another
region of embryo
• Deuterostomes develop the anus first from the blastopore
• Mouth develops later from another region of the embryo

19. 19
Evolution of the Animal Body Plan
Deuterostomes differ from protostomes in three other
fundamental embryological features:
1. Cleavage pattern of embryonic cells
̶ Protostomes = Spiral cleavage
̶ Deuterostomes = Radial cleavage
2. Developmental fate of cells
̶ Protostomes = Determinate development
̶ Deuterostomes = Indeterminate development

20. 20
Evolution of the Animal Body Plan
5. Evolution of segmentation
Segmentation provides two advantages:
• Allows redundant organ systems in adults (such as annelids, for
example)
• Allows for more efficient and flexible movement because each
segment can move independently
Segmentation appeared several times independently in
the evolution of animals – Annelida, Arthropoda,
Chordata

21. Homeostasis
• Homeostasis means to maintain dynamic equilibrium in the body.
• Ex: glucose, calcium, body temperature.
• Controlled by negative feedback loop.

22. Figure 33.20
• Blood sugar levels are controlled by a negative feedback loop. (credit: modification of work by Jon Sullivan)

23. Positive feedback loop
• Maintains the direction of the stimulus, possibly accelerating it.
• Ex: uterine contractions during childbirth

24. Figure 33.21
• The birth of a human infant is the result of positive feedback.

25. Figure 33.23
• The body is able to regulate temperature in response to signals from the nervous system.