Download Evolution of bilateral symmetry

General Features of Animals
• Animals are a diverse group that are characterized by several
features
– animals are heterotrophs
– all animals are multicellular and lack cell walls
– most animals are able to move from place to place
– most animals reproduce sexually
Five Transitions in Body Plan
•
1.
2.
The evolution of animals is marked by five key transitions to the body
Evolution of tissues
•
•
–
–
3.
only the sponges (Parazoa) lack both defined tissues and organs
all other animals (Eumetazoa) have true tissues
Evolution of bilateral symmetry
•
•
sponges also lack any definite symmetry and grow as irregular masses
virtually all of the Eumetazoa have a definite shape and symmetry
radial symmetry means the parts of their bodies are arranged around central axis
bilateral symmetry means that the body only has left and right mirror images
Evolution of a body cavity
•
a body cavity makes possible the evolution of efficient organ systems
•
4.
supports organs, distributes materials, and fosters complex developmental interactions
Evolution of deuterostome development
•
bilateral animals can be divided into two groups based on differences in the pattern
of embryonic development
•
•
5.
protostomes include flatworms, nematodes, mollusks, annelids, and arthropods
deuterostomes include echinoderms and chordates
Evolution of segmentation
•
segmented animals are assembled from a succession of similar segments
We will
know this
chart by
the end of
class today
Sponges: The Simplest Animals
• Sponges are members of the phylum Porifera
– They lack true tissues and organs but instead are
comprised of masses of specialized cells embedded in a
gel-like matrix
– the body of a sponge is perforated by tiny holes
• unique flagellated cells, called choanocytes, line the body cavity of
the sponge and draw water in through the pores
• the sponge is a “filter-feeder” that ingests any suspended food
particles trapped by the choanocytes as the water flows by
Cnidarians and Ctenophora
• All animals other than sponges have symmetry and tissues
• Two phyla exhibit radial symmetry (Radiata) - bodies are
organized around a central axis
• Cnidaria
– jellyfish, corals, and sea anemones
» Carnivores – first animals to use extracellular digestion – occurs in
gastrovascular cavity
» Stinging cells are called cnidocytes
» Each stinging cell or cnidocyte has a harpoon called a nematocyst
» Exist in two forms
• Medusae – free floating umbrella shaped
• Polyps – cylindrical shaped, usually attached to a rock
• Ctenophora
– comb jellies
Polyp
Medusae
Bilateral symmetry
• From here on out, all organisms we describe have
bilateral symmetry
• All bilateral animals have three embryonic layers that
develop into the tissues of the body
– Ectoderm, endorderm, mesoderm
Flatworms
• Flatworms are in the phylum
Platyhelminthes
• They have a gut with only one
opening – called the pharynx
– Food taken in and waste expelled
from here
• No other body cavity besides the
digestive system – acoelomates
• From here on out, all organisms have
a body cavity. Important advance for
3 reasons
– Circulation
– Movement
– Organ function
• Pseudocoelomate – body cavity
located between mesoderm and
endoderm
• Coelomate – body cavity located
within mesoderm
Pseudocoelomates: Nematoda and Rotifera
• All pseudocoelomates lack a defined
circulatory system, but most have a oneway digestive tract (meaning mouth and
anus now)
• In all pseudocoelomates, the pseudocoel
serves as a hydrostatic skeleton
• muscles can work against the rigidity of the fluidfilled cavity
• Nematodes
– unsegmented worms with a cylindrical (i.e.,
round) body
– covered by a flexible, thick cuticle that is shed
as they grow in a process called molting
– About 50 species of nematodes regularly
parasitize human beings
• Trichinella causes trichinosis after consuming
raw or undercooked pork
• From here on out, all organisms we discuss
are coelomates
Mollusks
• The only major phylum of coelomates without segmented bodies are the
mollusks (phylum Mollusca)
• Basic body design
• a head and foot
– the foot is muscular and may function in locomotion, attachment, or food capture
• a central visceral mass
– contains the body’s organs
• a mantle
– a heavy fold of tissue that surrounds the visceral mass
– respiratory organs are positioned on the inner surface of the mantle
• 3 major groups
– gastropods
• use the muscular foot to crawl and their mantle often secretes a single, hard protective cell
• all terrestrial mollusks are gastropods
– bivalves
• secrete a two-part shell with a hinge and filter-feed by drawing water into their shell
– cephalopods
• have a modified mantle cavity that permits jet propulsion and, in most groups, the shell is
reduced or absent
Pearls
• In mollusks with shells,
– the outer layer of the shell consists of
horny material that is rich in protein
– the inner layer of the shell consists of
pearly material that is used as
mother-of-pearl
• pearls are formed in clams and oysters
when a foreign object becomes lodged
between the mantle and the inner shell
• the mantle coats the foreign object with
layer upon layer of shell material to
reduce irritation
Annelids
• The first segmented animals to
evolve were the annelid worms
(phylum Annelida)
• Segmentation is the building of a body
from a series of similar segments
– a small change in an existing segment can
produce a new kind of segment with a
different function
– This allows for two important advancements
1.
specialized segments
•
2.
the anterior segments contain sensory organs and a
brain
connections
•
•
circulatory system is required because the segments
are divided by partitions
a nervous system allows for coordination
Arthropods
•
All arthropods have jointed
appendages, including legs, wings,
antennae, and mouthparts
–
•
Without jointed appendages, it is
impossible to walk, fly, or grasp an object
Arthropods also developed a rigid,
chitinous exoskeleton on the outside
of their body
–
–
provides points for muscle attachment
one drawback of a chitinous exoskeleton
was that it prevented arthropods from
reaching immense sizes
Embryo development
• Up to this point, all coelomates develop the
same way – called protostomes
• From here on out, organisms will develop in a
different way - deuterostomes
Echinoderms
• Echinoderms (phylum
Echinodermata) were the first
deuterostomes to evolve
– their name means “spiny skin,” which
refers to an endoskeleton composed
of hard, calcium-rich plates called
ossicles that lies just beneath the skin
– all echinoderms are bilaterally
symmetrical as larvae but become
radially symmetrical as adults
– a key innovation in echinoderms is a
hydraulic system that aids in
movement called the water vascular
system
• it is a system of fluid-filled canals that
extend into numerous, hollow tube-feet
Chordates
• Chordates (phylum Chordata) are deuterostome coelomates
characterized by a flexible rod called a notochord that
develops along the back of the embryo
• 4 principal features of chordates
1.
2.
3.
4.
•
•
•
•
notochord
a stiff but flexible rod that forms beneath the nerve cord in the early embryo
nerve cord
a single dorsal hollow nerve cord
pharyngeal pouches
a series of pouches behind the mouth that develop into slits in some animals
postanal tail
a tail that extends beyond the anus, at least during embryonic development
• All vertebrates are chordates
–
–
the only non-vertebrate chordates are the tunicates and the
lancelets
vertebrates are distinguished by having
•
•
backbone (comprised of a series of hollow bones) that replaces the
notochord
head with a skull and brain
Chordate phylogenetic tree
Fish
• Vertebrates first conquered the sea, then the land
• 4 characteristics that all fish share
1.
2.
3.
4.
gills

fish use gills to extract dissolved oxygen from water
vertebral column

all fishes have a skeleton made of either bone or cartilage
single-loop blood circulation

blood is pumped from the heart to the gills, to the body, and then back to the heart
nutritional deficiencies

fish are unable to synthesize the aromatic amino acids and must consume them in
their diet
 The first fishes to evolve were jawless
 Later, predatory fishes with jaws evolved, which eventually gave
rise to the sharks and bony fishes
 At this point, we are talking about various classes not phyla
anymore – all organisms are part of the chordata phylum
Sharks and rays
• Sharks and rays are members of the class
Chondrichthyes
• Sharks evolved into fast swimmers because
their cartilaginous skeleton is light and flexible
Bony fishes
• Bony fishes (class Osteichthyes) consist of
about 30,000 species
• Have a heavy internal skeleton of bone but are
still buoyant because of a swim bladder
– the swim bladder is a gas-filled sac that allows fish
to regulate their buoyant density
• Have a gill cover called the operculum – allows
them to breathe without having to move
Amphibians
•
Amphibians have five key
characteristics that allowed them
to successfully invade land
1.
2.
3.
4.
5.

legs
lungs
cutaneous respiration

supplements the use of lungs
pulmonary veins

returns oxygenated blood from the
lungs to the heart for repumping
partially divided heart

helps to prevent the mixing of oxygenated
blood with deoxygenated blood
must reproduce in water and are not
completely terrestrial
 many have a tadpole or




intermediate larval phase
Major groups include
Frogs and Toads
Salamanders and newts
Caecilians
Reptiles
• Reptiles replaced amphibians as the dominant terrestrial
vertebrates because of important features
– amniotic egg
• reptiles lay watertight eggs with various layers that protect and nourish the
embryo
– dry skin
• the skin of reptiles is covered by dry scales that prevent water loss
– thoracic breathing
• reptiles expand and contract the rib cage to move air in and out of the lungs
• Major groups
– Turtles and tortoises
– Crocodiles and alligators
– Snakes and lizards
Birds
• Birds evolved from small bipedal dinosaurs
– modern birds lack teeth and have only vestigial
tails
– they lay amniotic eggs and have reptilian scales on
their feet and lower legs
– birds are unique from reptiles in having
• feathers--lightweight and adapted for flight
• flight skeleton--rigid for attaching flight muscles; the
bones are thin and hollow
Mammals
• Mammals share three major characteristics
– mammary glands
• female mammals have mammary glands that produce milk to nurse the
newborns
– hair
• the primary function is insulation
• hair is made up of dead cells filled with the protein, keratin
– middle ear
• all mammals have three middle ear bones that evolved from bones in the
reptile jaw
• 3 major groups
– Monotremes – lay eggs, duck-billed platypus
– Marsupials - kangaroos
– placental mammals--most species of mammals today are placental
mammals