Evolution of the Animal Phyla
Chapter 19
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Outline
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Sponges
Cnidarians
Solid Worms
Roundworms
Mollusks
Annelids
Arthropods
Echinoderms
Chordates
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General Features of Animals
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All animals are multicellular heterotrophs.
Invertebrates account for 99% of all species.
Animal Kingdom includes about 36 phyla.
 Most occur in the sea.
 Three phyla dominate animal life on land.
- Arthropoda (spiders and insects)
- Mollusca (snails)
- Chordata (vertebrates)
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General Features of Animals
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Animal cells lack rigid cell walls and are
usually quite flexible.
Members of the animal kingdom tend to
move more rapidly and in more complex
ways than members of other kingdoms.
Most animals reproduce sexually.
 Embryonic Development
- Morula
- Blastula
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Phylogeny of Kingdom Animalia
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Sponges
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Kingdom Animalia has two subkingdoms:
 Parazoa - lack definite symmetry and
tissues organized into organs.
- Dominated by sponges (Porifera).
 Eumetazoa - have definite shape and and
symmetry and highly specialized cells.
Sponges are simplest animals.
 Bodies consist of masses of specialized
cells embedded in gel-like matrix.
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Sponges
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Phylum name, Porifera, refers to system of
pores found in body of sponges.
Flagellated cells, choanocytes (collar cells)
line body cavity.
 Draws water into body.
- Filter feeders
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Cnidarians
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Eumetazoans possess radial symmetrical
with ectoderm and endoderm. Two phyla:
 Cnidaria (Jellyfish, hydra, coral)
- Capture prey with tentacles.
 Cnidocytes (stinging cells)
 Nematocyst (harpoon)
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Cnidarians
Two body forms:
 Medusae - Free floating, gelatinous
 Polyps - Cylindrical
 Ctenophora (comb jellies)
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Radial vs. Bilateral Symmetry
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Radial Symmetry - Regular arrangement of
parts around central axis.
Bilateral Symmetry - Right and left halves
form mirror images.
 Dorsal vs. Ventral
 Anterior vs. Posterior
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Solid Worms
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Simplest of all bilaterally-symmetrical
animals.
 Ectoderm - Gives rise to outer covering
and nervous system.
 Mesoderm - Gives rise to digestive organs
and intestines.
 Endoderm - Gives rise to skeleton and
muscles.
Lack internal cavity, except for digestive tract
(acoelomate).
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Solid Worm Body Plan
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Solid Worms
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Flatworms
 Have incomplete gut and and cannot feed,
digest, and excrete food simultaneously.
- Excretory system consists of network of
tubules running through body.
 Lack circulatory system.
 Simple nervous system.
 Hermaphroditic
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Flatworm Anatomy
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Pseudocoelom vs. Coelom
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Evolution of body cavity was key transition.
 Circulation - Rapid passage of materials.
 Movement - Muscle-driven movement.
 Organ Function - Little deformation.
Acoelomates - No body cavity.
Pseudocoelomates - Possess pseudocoel.
Coelomates - Possess coelom (body cavity).
 Open circulatory system
 Closed circulatory system
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Bilaterally Symmetrical Body Plans
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Roundworms: Pseudocoelomates
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Seven Phyla
 Nematoda contains 12,000 recognized
species (nematodes and roundworms).
- Bilaterally symmetrical, cylindrical,
unsegmented worms.
 Covered by thick, flexible cuticle.
 Lack flagella or cilia.
 Reproduction is sexual.
- Trichinella - Live in pig intestines.
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Mollusks: Coelomates
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Development of specialized tissues involved
in primary induction in which one of three
primary tissues interacts with one another.
 Endoderm, Mesoderm, Ectoderm
Mollusks
 Only major phylum of coelomates without
segmented bodies.
 Body composed of three parts:
 Head-Foot, Visceral Mass, Mantle
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Mollusks: Coelomates
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Mollusks
 Gills capture oxygen from water and
release carbon dioxide.
 Radula - Rasping, tongue-like organ.
 Three Major Classes
- Gastropods
- Bivalves
- Cephalopods
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Annelids
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Key innovation to arise in coelomates was
segmentation.
 Building of body from series of similar
segments.
- Small change in existing segment can
produce new kind of segment with
different function.
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Annelids
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Worm within a tube design.
 Three characteristics:
- Repeated Segments
- Specialized Segments
- Connections
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Arthropods
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All have jointed appendages.
Rigid external skeleton (exoskeleton).
 Protects animal and provides places for
muscle attachment.
- Brittle, thus there is a limit to arthropod
body size due to exoskeleton thickness.
 Estimates of a quintillion insects alive at
any one time.
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Arthropods
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Chelicerates
 Arthropods lacking jaws.
- Mouthparts (chelicerae) evolved from
appendages nearest anterior end.
 Pediplaps
 Arachnida is largest class.
- 57,000 named species including spiders,
ticks, and scorpions.
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Chelicerates and Mandibulates
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Arthropods
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Mandibulates have mandibles formed by
appendages formed by other anterior
appendages.
 Crustaceans
- Differ from insects, but resemble
millipedes and centipedes as they have
legs on their abdomen as well as thorax.
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Crustaceans
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Arthropods
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Millipedes and Centipedes
 Consist of head region followed by
numerous similar segments. Centipedes
have one pair of legs on each segment,
and millipedes have two pairs of legs on
each segment.
Insects
 Largest group of arthropods.
- Head, Thorax, Abdomen
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Protostomes and Deuterostomes
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Protostome
 Mouth develops from or near blastopore.
Anus (if present) develops from another
region of embryo.
Dueterstomes
 Anus forms from, or near, blastopore, and
mouth forms subsequently on another part
of blastopore.
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Protostomes and Deuterostomes
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Deuterostomes differ from protostomes:
 Cleavage Pattern
- Spiral vs. Radial
 Developmental fate of cells
- Time of commitment
 Origination of coelom
- Mesoderm vs. Archenteron
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Embryonic Development in
Protostomes and Deuterstomes
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Echinoderms
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First Dueterstomes
 Endoskeleton composed of hard, calciumrich ossicles.
 Bilaterally symmetrical as larvae, but
become radially symmetrical as adults.
 Key evolution innovation was water
vascular system.
- Tube Feet
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Chordates
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Distinguished by four principle features:
 Notochord
 Nerve Cord
 Pharyngeal Slits
 Postnatal Tail
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Chordates
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Vertebrates
 Have internal skeleton made of bone or
cartilage against which the muscles work.
- Most endoskeletons made of bone.
 Bone formed in two stages:
 Matrix of collagen laid down.
 Minerals impregnate fibers.
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Vertebrate Embryonic Development
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Review
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Sponges
Cnidarians
Solid Worms
Roundworms
Mollusks
Annelids
Arthropods
Echinoderms
Chordates
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