Chapter 25

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Chapter 25
Invertebrates
A Summary
AP Biology
Spring 2011
Animals
Groups
Numbers
Chordates
Vertebrate chordates:
mammals, birds, reptiles,
amphibians, jawed fishes,
jawless fishes
Invertebrate chordates:
tunicates , lancelets
Vertebrates: 4500, 8600,
7000, 4900, 21000, 84
Invertebrates: 2100
Echinoderms
Sea stars, sea urchins
6,000
Arthropods
Crabs, spiders, insects
1,113,000 (recent
estimates of 10’s of
millions)
Roundworms
Pinworms, hookworms
22,000
Mollusks
Snails, slugs, octopuses
100,000
Annelids
Leeches, earthworms,
polychaetes
12,000
Rotifers
Bdelloids
2,000
Flatworms
Turbellarians, flukes,
tapeworms
20,000
Cnidarians
Jellyfish, hydra
10,000
Placozoans
Trichoplax adhaerens
1?
Poriferans
Sponges
9,000
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Animals: multicelled heterotrophs that
ingest other organisms or some portion of
them
Tissues are arranged into organs and organ
systems
Most motile at some stage of their life cycle
They reproduce sexually, and in some cases
asexually, or both
Embryos develop through a series of stages
Animal Origins

Variation in Animal Body Plans
◦ Organization:
◦ All animals are multicellular and show an
internal division of labor
◦ A key innovation for animals was development
of tissues
◦ Tissue development starts in the embryo
Specialized Cells, Tissues, Organs

Cell: the basic unit of all forms of life
◦ A neuron is a cell

Tissue: group of similar cells that
performs a particular function
◦ Sciatic nerve is a tissue

Organ: group of tissues that work
together to perform related functions
◦ Brain is a organ
Specialized Cells, Tissues, Organs

Tissue Development
◦ Outer ectoderm is the tissue giving rise to the
epithelium that lines the body surfaces and
forms other organs, and in some species,
serves as an external skeleton (ex. Insect
cuticle)
◦ Endoderm develops into the gut lining and
connective tissues serving as the structural
material for an internal skeleton
◦ Third tissue layer forms between the other two,
mesoderm, which is the source of muscles
and other organs in all animals more complex
than jellyfishes
Germ Layers
Germ Layers

Diploblastic: animals with only 2 cell
layers, their bodies consist of ectoderm
and endoderm, and mesoglea (middle
glue) holds two layers together
◦ Porifera and cnidarians

Triploblastic: having 3 true cell layers
◦ Complex animal phyla
Germ Layers

Bilateral Symmetry: body is organized
along a longitudinal axis with right and
left sides that mirror each other
◦ Most are triploblastic
◦ Occurs in humans, lobsters

Radial Symmetry: any cut through the
central axis of the organism would
produce mirror images
◦ Occurs in jellies
Body Symmetry
Symmetry

Cephalization: concentration of sensory
equipment at one end (usually the
anterior, or head end) of the organism
Development of Head

Coelomates: posses a body cavity filled
with fluid, and this space separates an
animal’s digestive tract from the outer
body wall
◦ A true coelom forms from tissue derived from
mesoderm
◦ Most complex: Annelida, Mollusca, Arthropoda,
Chordata
Body Plans

Pseudocoelomates: are triploblastic
animals with a cavity formed from the
blastocoel, rather than the mesoderm
◦ Nematodes or roundworms

Acoelomates: have no cavities between
their alimentary canal and the outer wall
of their bodies
◦ Flatworms
Body Plans

Coelom: fluid filled body cavity that is
completely surrounded by mesoderm
tissue
◦ Represents advance in course of animal
evolution because it provides a space for
elaborate organ systems
◦ Like digestive tract or cardiovascular system
Body Plans
From the Primitive
To the Complex
No symmetry or radial symmetry
Bilateral symmetry
No head
Head with sensory apparatus
Mesoglea holds two cell layers
together
3 cell layers, including mesoderm
Acoelomate
Pseudocoelomate or coelomate
No true tissue
True tissues, organs, and organ
systems
Little specialization
Much specialization
Sessile
Motile
Be familiar with 9 common phyla and
representative animals of each
 For each animal phyla: think in terms of
strategies that animals have evolved to
adapt to particular environments and
notice the trends of development from
simple to complex

Characteristics of Animals
Domain
Archaea
Bacteria
Eukarya
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No symmetry
Have no nerve of muscle tissues
Sessile- do not move
Filter nutrients from water drawn into a
central cavity
Consist of two cell layers only: ectoderm
and endoderm connected by noncellular
mesoglea
Porifera: Sponges
Have specialized cells but no true tissues
or organs, each cell carriers out many
functions
 Evolved from colonial organisms

◦ If you squeeze a sponge through fine
cheesecloth it will separate into individual cells
that will spontaneously reaggregate into a
sponge
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Reproduce asexually by fragmentation
Also reproduce sexually: hermaphrodites
Porifera: Sponges
Radial symmetry
 Body plan is polyp (vase shaped), which
is mostly sessile, or medusa (upside
down bowl shaped), which is mostly
motile
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Cnidarians: Hydra and Jellyfish

Life cycle: some go through a planula
larva (free swimming) stage then go
through two reproductive stages
◦ Asexually reproducing (polyp)
◦ Sexually reproducing (medusa)
Cnidarians: Hydra and Jellyfish
Two cell layers only: ectoderm and
endoderm connected by noncellular
mesoglea
 Have a gastrovascular cavity where
extracellular digestion occurs
 Carry out intracellular digestion inside
body cells in lysosomes

Cnidarians: Hydra and Jellyfish
Have no transport system because every
cell is in direct contact with the
environment
 All members have stinging cellscnidoctes

◦ Containing stingers, which are called
nematocysts

Ex. Hydra
Cnidarians: Hydra and Jellyfish

Simplest animals with:
◦ Bilateral symmetry
◦ An anterior end and three distinct cel layers
(ectoderm, endoderm, mesoderm)
Platyhelminthes: Flatworms,
Tapeworms
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Digestive cavity has only one opening for
both ingestion and egestion so food cannot
be processed continuously
Have solid body and no room for true
digestive or respiratory systems to circulate
food or oxygen
Flatworms have solved this problem in a
unique way
They body is so flat and thin that many body
cells can exchange nutrients and wastes by
diffusion with the environment
Platyhelminthes: Flatworms,
Tapeworms
Unsegmented worms with bilateral
symmetry but little sensory apparatus
 Many are parasitic

◦ Trichinella causes trichinosis, contracted by
eating uncooked pork

C. elegans: widely used as an animal
model in studying genes and embryonic
development
Nematodes: Roundworms
C. elegans
Platyhelminthes: Flatworms,
Tapeworms
Bilateral symmetry with little sensory
apparatus
 Digestive tract is a tube-within-a-tube
consisting of crop, gizzard, and intestine
 Nephridia for excretion of nitrogen
wastes, urea
 Closed circulatory system: heart consists
of 5 pairs of aortic arches

Annelids: Segmented Worms
(earthworms, leeches)
Blood contains hemoglobin and carries
oxygen
 Diffusion of oxygen and carbon dioxide
through moist skin
 Hermaphrodites
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Annelids: Segmented Worms
(earthworms, leeches)
Have soft body often produced by a hard
calcium-containing shell
 Open circulatory system with blood-filled
spaces called hemocoels or sinuses
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Mollusks: Squids, Octopuses,
Slugs, Clams, and Snails

Have bilateral symmetry with 3 body
zones:
◦ Head-foot: contains both sensory and motor
organs
◦ Visceral mass: contains the organs of
digestion, excretion, and reproduction
◦ Mantle: specialized tissue that surrounds the
visceral mass and secretes the shell
Mollusks: Squids, Octopuses,
Slugs, Clams, and Snails
Radula: movable, tooth-bearing
structure, acts like a tongue
 Most have gills and nephridia
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Mollusks: Squids, Octopuses,
Slugs, Clams, and Snails
Jointed appendages
 Segmented into head, thorax, abdomen
 More sensory apparatus than in annelids
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◦ Gives them more speed and freedom of
movement
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Chitinous exoskeleton protects animal and
aids in movement
Arthropods: Insecta
(grasshopper), Crustacea (shrimp,
crab), Arachnida (spider)
Open circulatory system with a tubular
heart and hemocoels, sinuses
 Malpighian tubules for removal of
nitrogenous wastes, uric acid
 Air ducts called trachea bring air from the
environment into hemocoels
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Arthropods: Insecta
(grasshopper), Crustacea (shrimp,
crab), Arachnida (spider)
Most are sessile or slow moving
 They have bilateral symmetry as an
embryo but revert to the primitive radial
symmetry as an adult
 The radial anatomy of the adult is an
adaptation to a sedentary lifestyle
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Echinoderms: Sea Stars (Starfish)
and Sea Urchins
Their water vascular system creates
hydrostatic support for the tube feet, the
locomotive structures
 Reproduce by sexual reproduction with
external fertilization
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Echinoderms: Sea Stars (Starfish)
and Sea Urchins
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Can reproduce by fragmentation and
regenerate
◦ Any piece of a sea star that contains part of the
central nervous canal will form a completely
new organism

Have endoskeleton consisting of calcium
plates
◦ Endoskeleton grows with the body
◦ Exoskeleton does not and must be shed
periodically
Echinoderms: Sea Stars (Starfish)
and Sea Urchins
Two subphyla of invertebrate chordates
 Simpler versions of vertebrates
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Chordates: Tunicates, Lancelets
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Many of 4 anatomical features that
characterize chordates appear only during
embryonic development
◦ Notochord: long, flexible rod that appears
during embryonic development between the
digestive tube and the dorsal nerve cord
◦ Dorsal, hollow nerve cord: formed from a plate
of ectoderm that rolls into a hollow tube
Chordates: Tunicates, Lancelets
◦ Pharyngeal clefts: grooves that separate a
series of pouches along the sides of the
pharynx; in most chordates the clefts develop
into slits that allow water to enter and exit he
mouth without going through the digestive
tract
◦ A muscular tail posterior to the anus
Chordates: Tunicates, Lancelets
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Have a notochord: rod that extends the
length of the body and serves as a flexible
axis
Have a dorsal, hollow nerve cord
The tail aids in movement and balance
Coccyx bone in humans is a vestige of a
tail
Vertebrates
Chordates: Fish, Amphibians,
Reptiles, Birds, Mammals

Birds and mammals are hemeotherms
◦ Maintain a consistent body temperature
◦ All other chordates: fish, amphibians, and
reptiles
 Cold blooded although some reptiles are
endotherms (heat from within) and are able to
raise their body temperature
Chordates: Fish, Amphibians,
Reptiles, Birds, Mammals

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