Remember…all animals are “multicellular,
heterotrophic eukaryotes”. 
In most animals, cells interact at three levels of
organization.
 tissues
 organs
 organ systems
Animal bodies are organized at the basic level of the cell,
which in combination (of similar cells) creates tissues,
which work together to form organs, which work together
to form organ systems…which make up the organism.
• Epithelial tissue
covers the body’s
surface and lines its
internal tubes.
• Connective tissue
provides support and Epithelial Tissue
connects body parts.
Connective Tissue
• Muscle tissue moves
Nervous Tissue
the body and its parts.
Muscle Tissue
• Nervous tissue
detects internal and
external stimuli and
coordinates responses.
Parazoa
• Literally translates “besides the animals”
• Unlike true animals (or Eumetazoa) these organisms
have no distinct tissues.
The only surviving parazoans are
the sponges, which belong to the
phylum Porifera, and one
surviving species (Trichoplax
adhaerens) in the phylum
Placozoa
Organs and Organ
Systems
Organs and organ systems help
animals to organize and
compartmentalize the tasks of
survival and reproduction for the
body as a whole. Organs may
belong to multiple systems.
The primary tissue layers that
form in the early embryo
(ectoderm, mesoderm, and
endoderm) give rise to various
organs and organ systems.
Three tissue (germ)
layers that form
during gastrulation
develop into many
different tissues.
• ectoderm
• mesoderm
• endoderm
Organisms with
three tissue layers
of development
are triploblastic.
Organisms with
only two tissue
layers of
development are
diploblastic.
Tissue formation begins in an embryo. The simplest
animals are diploblastic. They have only ectoderm,
and endoderm.
Cnidaria are invertebrates
that are diploblastic.
What similarities
do you notice
about all these
animals?
Platyhelminthes are
another invertebrate
phyla with diploblastic
development.
When embryonic development
involves the formation of three
tissue layers, (as is the case with
most animals) we have
triploblastic development.
Most animal embryonic
development follows this
plan:
• Zygote (fertilized egg)
• Cleavage (division to 8 cells)
• Blastula (hollow ball stage)
• Gastrulation (pinching in of
blastula, to form blastopore)
• Gastrula formation, and
independent tissue formation.
In animals, the dominant generation is 2n. What
generation would you compare this to in the plant
kingdom, and why?
Sporophyte; gametophyte is (n), and
sporophyte is 2n
What would the only exception to that rule be in the
plant kingdom?
Bryophytes (mosses),which have a
dominant n (gametophyte) generation.
Radially symmetrical animals
(such as coral and jelly fish)
have body parts organized
about a central axis and tend
to be cylindrical in shape.
This feature is normally
Asymmetrical animals
associated with a sedentary
(sponges) have no
general body plan or axis or sessile life style, rather
of symmetry that divides than motile.
the body into mirrorimage halves.
Bilateral Symmetry in
animals (such as
humans and fish)
have only a single
plane of symmetry
that produces mirror
halves.
• Organisms that are radially symmetrical have only a top and a bottom (or
front and back).
• Organisms that are bilaterally symmetrical have a top and bottom (dorsal,
ventral), and a head and tail end (anterior, posterior)
Whenever you see this
word…think “head”.
Increased cephalization
occurs in higher level
animals with bilateral
symmetry, from a
collection of ganglia
(nervous tissue) to brains.
What type of body plan, or symmetry does a sea
anemone exhibit?
Raidal
Does a sea anemone
exhibit cephalization?
No…animals without
bilateral symmetry
do not have “heads”.
Name an organism that exhibits bilateral symmetry,
and prove it by naming its planes of symmetry, (or
surfaces). Dogs (any vertebrate). Dorsal, ventral,
anterior, posterior
Whenever you see this phrase,
think “guts”. Simple animals
have a mouth that doubles as an
anus. In more advanced
organisms the cavity is through.
Acoelomate animals,
like Platyhelminthes,
have no body cavity at
all. Organs have direct
contact with the
epithelium. Semi-solid
mesodermal tissues
between the gut and
body wall hold their
organs in place.
Pseudocoelomate animals have a pseudocoel, which is a fully
functional body cavity. Tissue derived from mesoderm only partly
lines the fluid filled body cavity of these animals. Thus, although
organs are held in place loosely, they are not as well organized as in
a eucoelomate. (true coelomate)
Nematodes and Rotifers (two
invertebrate phyla) are the only
pseudocoelomate animals. These also
represent the first animals of
triploblastic development.
Eucoelomates have a fluid filled body cavity called a coelom with a
complete lining called peritoneum derived from mesoderm. The
complete mesoderm lining allows organs to be attached to each
other so that they can be suspended in a particular order while
still being able to move freely within the cavity. Most animals with
bilateral symmetry are coelomates. This fluid filled sac allows for
the growth and movement of internal organs.
Some cells migrate upward
into the blastula to form a
blastopore. This opening
eventually forms the mouth of
animals called protostomes.
If the opening does not form
the mouth, but instead forms
the anus, the animals are
deuterostomes.
During embryonic development, many organisms show
tendencies to either be protostomes, or deuterostomes.
Deuterostomes are the more complex of the
organisms
Two major lineages of bilaterally symmetrical animals
differ in how their digestive systems form.
• Protostomes: blastopore becomes a mouth
Platyhelminthes
Nematodes
Rotifers
Annelids
Arthropods
Mollusks
• Deuterostomes: blastopore becomes an anus
Echinoderms
Chordates/Vertebrates
Only a few animals are able to
reproduce asexually. When it
occurs, asexual reproduction in
animals usually occurs in one of
three ways:
• Parthenogenesis: Modified sexual
reproduction, in which eggs develop
without fertilization
•Polyembryony: internal production of
propagules, which are released and
develop into new organisms (identical
twins)
•Budding: splitting to produce new
individuals