What is an Animal?
Chapter 25 TURBO BLAST
Why are animals classified as one
kingdom?
• All animals are:
– Eukaryotic, multicellular
– Able to move in specific ways to obtain food,
reproduce, and protect themselves
• Most animals:
– Have specialized cells/tissues (that form organs)
How do animals obtain food?
• All animals are heterotrophic (must eat other
organisms for food)
• Depend either directly or indirectly on
autotrophs for food
• Animals hypothetically evolved in water (recall
what cells require for all biochemistry)
• Some animals living in water do not move
from place to place, must filter food from
water
– Organisms that permanently attach to a
surface are called sessile
– This is not possible for land animals—
there is very little suspended food in the air
– Land animals need to use more oxygen and energy
to find food
How do animals digest food?
Animals must ingest (take in) food
– Once ingested, food must be digested
• Some animals digest food within special cells
• Other animals have a special internal digestion cavity
– Food is not always available for animals; when an animal
encounters excess food, it can be stored as fat or glycogen to
use as fuel when food is not available
• Ex: Flatworms (planarian) have only one opening to their digestive
tract, called a pharynx
• Ex: Earthworms have a mouth (entrance) at one end and an anus
(exit) at the other end of the tract
What are the functions of some animal
cells?
• Most animal cells are specialized to carry out
different functions
– Sensing food
– Finding mates
– Identify predators/protect themselves
– Light reception
– Homeostasis
– Movement
Development and Fertilization
• Most animals reproduce sexually (males
produce sperm, females produce eggs)
– Some animals do reproduce asexually
• Most animals develop from a fertilized egg cell
called a zygote
– Sperm (n) + egg (n) = zygote (2n)
• Fertilization may be internal or external for
different species
• After fertilization the development of different
animal species all have similar, genetially
determined stages of development
Embryonic Development
• The zygote (single fertilized cell) divides by
mitosis and cell division to form two cells
– This process is called cleavage
– Once cell division has occurred, the organism is
known as an embryo
A: zygote
B: two cells
C: four cells
D: 8 cells
E: continued cell divisions…
F: cell-covered, fluid-filled
ball called a blastula
*blastula is formed early in
development
Gastrulation
• After blastula is formed, cell division continues
– Cells from one side of blastula move inward to form a
gastrula
• Gastrula: structure made up of two layers of cells with an
opening at one end
(Picture how a potter might make a bowl from a round ball of clay)
– Layer of cells on outer surface of gastrula is called the
ectoderm – eventually becomes skin and nervous tissue
– Layer of cells lining inner surface of gastrula is called the
endoderm—eventually becomes digestive tract
How is the mesoderm formed?
• In some animals, development of the gastrula
progresses until a layer of cells called the
mesoderm forms
– Mesoderm is the third cell layer found in
developing embryo, found between endoderm
and ectoderm—eventually becomes muscles,
circulatory, excretory, and respiratory systems
Which came first: the mouth or the anus?
• When the opening in the gastrula develops into the
mouth, the animal is a protostome
– (proto = first; stoma = mouth)
– Ex: snails, earthworms, insects
• When the opening in the gastrula develops into the
anus first, the animal is a deuterostome
– (deutero = second; stoma = mouth)
– Ex: starfish, fish, amphibians, birds, reptiles, and you
• Scientists hypothesize that protostomes appeared first
in evolutionary history, followed by deuterostomes
• Determining if an animal is a protostome or
deuterostome can help biologists identify its
phylogeny
What changes occur during growth an development?
• Embryonic development of animals holds clues to
animal evolutionary history
• Cells in developing embryos continue to
differentiate and become specialized to perform
different functions
– Most animal embryos develop into juveniles (look like
smaller versions of the adult)
– In some animals (insects, echinoderms), the embryo
develops inside an egg into a intermediate stage called a
larva—usually doesn’t look like the adult
• Sessile animals usually have free-moving larvae
• Larvae continue to grow into juveniles, then to adults
Body Plans and Adaptations
• Animal body structure is related to how the animal
moves around during its lifetime
– Symmetry – describes how an animal’s body structures
are arranged
• Asymmetry—no line can divide into two equal
halves
– The bodies of most sponges consist of two layers of cells
(no formation of endoderm, mesoderm, or gastrula)
Body Symmetry Relates to Animal
Movement
• Radial symmetry can be divided along any plane
through a central axis into roughly equal halves
(allows animals to move in 360⁰ and detect and
capture prey from any direction)
– Animals with radial symmetry include: hydras, jellyfish, starfish,
sea anemone, coral, sand dollars
– Animals with radial symmetry have muscle movement
controlled by a neural net
Body Symmetry Relates to Animal
Movement
• Bilateral Symmetry - can be divided
down the animal’s length into
similar right and left halves
– Animals with bilateral symmetry
move forward, have more
coordinated movement
– Anterior or head end has sensory
organs (vision, olfactory, auditory)
and a central nervous system
– Posterior is the tail end
– Dorsal is the upper surface, which
looks different from the Ventral or
lower surface
Bilaterally Symmetrical Animals All
Develop from Three Embryonic Layers
• Recall the ectoderm, mesoderm, and
endoderm cell layers - these give rise to
specialized regions in animal body plans
– Some bilaterally symm. Animals also have a fluidfilled cavity called a coelom (coel = “cavity”) that
provides internal support for movement
– Animals that lack a coelom are acoelomates
– Animals that have a coelom partially covered in
mesoderm are pseudocoelomates
– Animals with a coelom surrounded by mesoderm
are coelomates
• A coelom acts as leverage for muscle
contractions
• Think of the rigidity of a very-full water
balloon
• Leads to flexibility of the organism
(bending from side to side, squeezing
in sections, etc.)
• Acoelomates may have been the first
organisms to evolve
• Have a digestive tract that extends
throughout the body
• Organs are embedded in the solid
tissues of the body
• Pseudocoelomates have more complex
movement
• Also have a one-way digestive tract
(mouth, middle section for food
breakdown/absorption, anus)
• Coelomates have the greatest diversity
• Specialized organs and organ systems
develop in the coelom (which cushions
organs and allows them to grow &
move within body cavity)
Body cavities led to greater animal diversity
• As animals diversified, they adapted to life in different
environments
– Some (like mollusks) developed hard shells to protect soft
bodies
– Some developed hardened internal support, such as spicules
(like sponges)
– Some developed exoskeletons – hard coverings on the
outside of the body that provide a framework, protect soft
tissues, prevent water loss, and provide protection from
predators
• These are rigid and must be shed for the animal to grow larger
• Exoskeletons are often possessed by invertebrates, animals that lack
a spine (ex: crabs, beetles, spiders)
– Some developed endoskeletons – internal skeletons that
protect internal organs and provide brace for muscles
• Endoskeletons are made of calcium carbonate, cartilage, or bone
– (Starfish are invertebrates with endoskeletons, while fish, reptiles,
amphibians, birds and mammals are vertebrates)
• A vertebrate is an animal with an endoskeleton and a
backbone (spine)
– All are bilaterally symmetrical
Invertebrate
Invertebrate with
Endoskeleton
Invertebrate
with Spicules
Invertebrate with
Exoskeleton
Vertebrates with Endoskeleton
and Spine
Animal Origins
• Animals likely evolved from aquatic, colonial
protists
– Traced back to late Precambrian fossil records
– Bilateral symmetry appears much later, but all body
plans that exist today were present by the Cambrian
Period 543 million years ago
• These different body plans have diversified since then, but no
new body plans have arisen (based on fossil evidence)