Evolution of the Animal
Body Plan
Part 1 - Invertebrate Animals
E. Q. – How has the animal body plan
changed over time?
Phylum Porifera (Sponges)
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•
•
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- Development of Multicellularity
Sponges are the simplest
• Once food has been ingested
animals. Their bodies consist
by the choanocytes, it passes
of a few types of cells grouped
to wandering amoebocytes by
together.
diffusion. The amoebocytes
move through the rest of the
They do not have symmetry,
sponge, where the food passes
germ layers, or any other
to remaining cells again by
characteristics of more highly
diffusion.
evolved animals.
Sponges are filter feeders that • Sponges maintain their shapes
because they have stiff fibers
live off of bacteria and other
(spicules) or flexible fibers
microscopic organisms.
(spongin).
Water flows through the body
• Sponges reproduce asexually
of the sponge past
or sexually.
choanocytes, also called collar
cells. These cells trap the food • Sponges are mostly marine,
particles and ingest them by
but there are a few freshwater
phagocytosis.
species.
Phylum Porifera
Phylum Cnidaria (Hydras, Sea
Anemones, Jellyfish, Corals)
- Development of tissue layers and symmetry
• Most cnidarians are marine,
but there are also some
freshwater species, such as
hydras.
• Most are sessile or slowmoving, but are excellent
predators.
• Unlike Poriferans, cnidarians
have symmetry (radial), two
germ layers (endoderm and
ectoderm), and a single
digestive opening surrounded
by tentacles carrying stinging
cells (cnidocytes).
• Cnidarians have two basic
body plans—polyp or medusa.
• Because they are radially
symmetrical, cnidarians do
not have a head.
• The Cnidarian’s central body
cavity is its gastrovascular
cavity, which allows the
animal to eat prey larger than
microscopic organisms.
• Cnidocytes, another phylum
characteristic, contain a
venomous barb that paralyzes
prey. The tentacles then pull
the prey into the mouth.
Phylum Cnidaria
Phylum Cnidaria
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Phylum Platyhelminthes
(Planaria, Flukes, Tapeworms)
- Development of bilateral symmetry, cephalization, and organs
• There are about 20,000 species •
of Phylum Platyhelminthes.
They inhabit freshwater and
marine environments and
•
include free-living as well as
parasitic forms.
• One of the most familiar
•
platyhelminths is the free-living
planarian Dugesia, which feeds
along rock surfaces and the
undersides of vegetation.
• The animal is acoelomate like
cnidarians. It does exhibit
major evolutionary advances,
however. Platyhelminths are
bilaterally symmetrical, and
have evidence of cephalization.
They have three germ layers
and distinct organs and organ
systems.
They reproduce asexually and
sexually. Most platyhelminths
are hermaphrodites.
Tapeworms are adapted to a
parasitic life. They lack
digestive cavities or a means
of locomotion, but do have an
organ of attachment, the
scolex. They also produce
abundant numbers of
offspring, since each of their
many posterior segments
(proglottids) contains ovaries
and testes.
Phylum Platyhelminthes
Phylum Nematoda
- First sign of a body cavity ( pseudocoelom)
• The phylum Nematoda is made
of roundworms, which are
commonly known as
nematodes. Some are
parasitic, such as roundworms
and hookworms.
• One, Trichonella sporalis,
which can be transmitted to
humans by undercooked meat,
causes the disease trichinosis.
• Many others are free living,
such as nematodes found in
soil and vinegar eels.
• All are bilaterally symmetrical,
and have similar body plans,
that show evolutionary
advances over platyhelminths.
• Nematodes have some degree
of cephalization, with two
nerve cords.
• They also have two openings
to the digestive tract, which
allows the food to move in
one direction through the gut.
• Nematodes have a body
cavity that forms between the
endoderm and the mesoderm
called a pseudocoelom. This
allows for independent
movement of the gut and the
body wall.
• Nematodes reproduce
sexually, and the sexes are
separate. Fertilization is
internal, and thousands of
offspring are produced.
Phylum Nematoda
Phylum Mollusca
- Development of a true coelom
• This phylum includes clams,
oysters( bivalves), snails and
slugs (gastropods) , and octopi
and squids (cephalopods).
• These animals are coelomate
but unsegmented, with soft
bodies. They have three
general body regions: the
mantle, the foot, and the
visceral mass.
• All mollusks have a strong
muscular foot (different in
different mollusks), used for
creeping, swimming, or
catching prey.
• Many mollusks have an external
shell, which is secreted by their
mantle.
• Most mollusks are aquatic, but
some are terrestrial. In
aquatic mollusks, gills are
used for respiration.
• Most mollusks have an open
circulatory system with a
heart.
• Bivalves are filter feeders,
with powerful adductor
muscles that hold their two
shells together.
• Cephalopods have feet
modified into tentacles and
well-developed nervous
systems.
• Mollusks other than bivalves
have a serrated radula used
to break up large food
particles.
Phylum Mollusca
Phylum Annelida
- Development of body segmentation
• Members of the phylum
Annelida include many marine
worms, earthworms, and
leeches.
• Their major evolutionary
advance is that their bodies
are segmented. They have a
head with a mouth, and a
terminal segment with an
anus.
• They also have a true coelom,
which develops completely
within the mesoderm.
• Their digestive tract runs the
length of the worm and
contains specialized organs
such as the pharynx,
esophagus, crop, gizzard, and
intestine.
• Annelids have segmental
bristles (setae) and both
longitudinal and circular
muscles lining the coelom,
which are used for swimming,
crawling, or burrowing.
• Annelids have complex,
closed circulatory systems,
with multiple muscular pumps
(hearts).
• Their nervous systems have a
simple brain and a ventral
nerve cord that connects it to
other areas of the body.
• The annelids have nephridia
in each segment, which
excrete nitrogenous wastes.
Phylum Annelida
Phylum Arthropoda (Crustacea)
- Development of jointed appendages, specialized segments
• Arthropods are the most
successful animal phylum.
Most animals on Earth are
arthropods. There are over
1,000,000 different species.
• An arthropod has a
segmented body with a
chitinous exoskeleton and
jointed appendages.
• In order to grow, arthropods
must shed their exoskeleton
periodically.
• Their segments do not repeat,
like those of annelids, but
instead are highly specialized
for feeding, locomotion,
reproduction, and sensing the
environment.
• An arthropod has an open
circulatory system with a welldeveloped digestive system.
• Phylum Arthropoda includes
millipedes and centipedes,
crustaceans, arachnids, and
insects.
• Most crustaceans are aquatic, but
a few, like the rolly-polly are
terrestrial.
• Other crustaceans are crabs,
lobsters, crayfish and shrimp.
• Crustaceans generally have five
pairs of walking legs and two pairs
of antennae.
• Crustaceans have mineralized
exoskeletons, which make them
sturdier than the exoskeletons of
other arthropods.
Phylum Arthropoda (Spiders,
Scorpions, Millipedes, Centipedes)
• Millipedes and centipedes have
• Arthropods that have eight
cylindrical bodies made of many
legs include spiders,
repeating segments with distinct
scorpions, ticks, and mites.
heads and posteriors and a
They are also called
chitinous exoskeleton.
arachnids. In arachnids, the
head and the thorax are
• Millipedes have two pair of jointed
usually fused together. Many
walking legs per body segment,
species are arachnids are
are non-venomous, and eat
venomous. Ticks are blooddecaying plant matter.
sucking parasites that are
• Centipedes are similar in
vectors for many diseases.
appearance, but have only one
pair of walking legs per segment. • Scorpions are one of the
earliest terrestrial animals.
• They are predators and
scavengers. They are venomous. • Spiders breathe using a book
lung or tracheae, and can spin
Their first pair of legs is used to
silk.
inject poison into their prey.
Phylum Arthropoda (Insecta)
• Many of the adaptations of
insects that have made them
so successful are also
characteristic of other
arthropods.
• All insects have a segmented
body with an exoskeleton of
chitin and jointed appendages
specialized for various uses.
• The body of an insect is
divided into three regions: the
head, the thorax, and the
abdomen.
• All insects have three pairs of
walking legs, which are
attached to the thorax. If
wings are present, they are
also on the thorax.
• The abdomen contains 9-11
segments, and in adults, it has
no legs nor wings attached.
• Many insects, like other
arthropods, have compound
as well as simple eyes.
• An insect has an open
circulatory system with a
muscular heart to circulate
nutrients.
• Gas exchange is done by the
tracheae and spiracles.
• Insects and spiders use
Malpighian tubules to excrete
metabolic wastes.
• Insects have a brain and
ventral nerve cord.
Phylum Arthropoda
Phylum Echinodermata
- Development of deuterostomy in place of protostomy
• Echinoderms have spiny skins
that are made of hard,
calcified plates that form a
kind of endoskeleton just
below the epidermis.
• All echinoderms live in
saltwater or brackish water.
• They are radially symmetrical
as adults but bilaterally
symmetrical as larvae.
• Echinoderms are not
segmented, and they lack a
head.
• Echinoderms have a modified
coelom called the water
vascular system that helps
them move.
• When an echinoderm feeds, it
extends its gut outside of its
body.
• Sexes are separate in
echinoderms, and fertilization
is external.
• Echinoderms have no
centralized nervous system.
• Examples of echinoderms
include starfish, sea urchins,
sand dollars, brittle stars, and
sea cucumbers.
• Echinoderms are
deuterostomes, unlike the
other invertebrate phyla.
• Phylum Chordata is the most
advanced animal phylum.
Phylum Echinodermata
E. Q. – How has the animal body plan
changed over time?
With respect to each of the following concepts, tell how
the animal body plan has evolved:
Tissue layers
Type of symmetry
Cephalization
Development of organ systems
Body cavity
Body segmentation
Appendages
Embryonic development
Nervous system development