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Chapter 33
Invertebrates
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Overview:
• Invertebrates
– Are animals that lack a backbone
– Account for 95% of known animal species
Figure 33.1 Christmas Tree Worm
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Chordata
Echinodermata
Other bilaterians (including
Nematoda, Arthropoda,
Mollusca, and Annelida)
Porifera
Cnidaria
• A review of animal phylogeny
Deuterostomia
Bilateria
Eumetazoa
Ancestral colonial
choanoflagellate
Figure 33.2
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Exploring invertebrate diversity
PORIFERA (5,500 species)
A sponge
PLACOZOA (1 species)
CNIDARIA (10,000 species)
A jelly
KINORHYNCHA (150 species)
0.5 mm
250 µm
A placozoan (LM) A kinorhynch (LM)
ROTIFERA (1,800 species)
PLATYHELMINTHES (20,000 species)
A marine flatworm
ECTOPROCTA (4,500 species)
A rotifer (LM)
PHORONIDA (20 species)
Ectoprocts Phoronids
Figure 33.3
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Exploring invertebrate diversity
BRACHIOPODA (335 species)
A brachiopod
ACANTHOCEPHALA (1,100 species)
NEMERTEA (900 species)
A ribbon worm
CTENOPHORA (100 species)
5 mm
An acanthocephalan
MOLLUSCA (93,000 species)
A ctenophore, or comb jelly
ANNELIDA (16,500 species)
An octopus
LORICIFERA (10 species)
A marine annelid
PRIAPULA (16 species)
50 µm
Figure 33.3
A loriciferan (LM)
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A priapulan
Exploring invertebrate diversity
ARTHROPODA (1,000,000 + species)
NEMATODA (25,000 species)
A roundworm
A scorpion (an arachnid)
CYCLIOPHORA (1 species)
TARDIGRADA (800 species)
100 µm
100 µm
A cycliophoran (colorized SEM) Tardigrades (colorized SEM)
HEMICHORDATA (85 species)
ONYCHOPHORA (110 species)
An onychophoran
An acorn worm
ECHINODERMATA (7,000 species)
Figure 33.3
A sea urchin
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CHORDATA (52,000 species)
A tunicate
Concept 33.1:
• Sponges, phylum Porifera, are sessile and have a
porous body and choanocytes
• Sponges
– Live in both fresh and marine waters
– Lack true tissues and organs
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• Sponges are suspension feeders
– Capturing food particles suspended in the
water that passes through their body
5 Choanocytes. The spongocoel
is lined with feeding cells called
choanocytes. By beating flagella,
the choanocytes create a current that
draws water in through the porocytes.
Azure vase sponge (Callyspongia
plicifera)
4 Spongocoel. Water
passing through porocytes
enters a cavity called the
spongocoel.
3 Porocytes. Water enters
the epidermis through
channels formed by
porocytes, doughnut-shaped
cells that span the body wall.
2 Epidermis. The outer
layer consists of tightly
packed epidermal cells.
Figure 33.4
Flagellum
Collar
Food particles
in mucus
Choanocyte
Osculum
Phagocytosis of
food particles
Spicules
Water
flow
1 Mesohyl. The wall of this
simple sponge consists of
two layers of cells separated
by a gelatinous matrix, the
mesohyl (“middle matter”).
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Amoebocyte
6 The movement of the choanocyte
flagella also draws water through its
collar of fingerlike projections. Food
particles are trapped in the mucus
coating the projections, engulfed by
phagocytosis, and either digested or
transferred to amoebocytes.
7 Amoebocyte. Amoebocytes
transport nutrients to other cells of
the sponge body and also produce
materials for skeletal fibers (spicules).
• Choanocytes, flagellated collar cells
– Generate a water current through the sponge
and ingest suspended food
• Most sponges are hermaphrodites
– Meaning that each individual functions as both
male and female
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Concept 33.2: Cnidarians
• Cnidarians have radial symmetry, a gastrovascular
cavity, and cnidocytes
• All animals except sponges
– Belong to the clade Eumetazoa, the animals
with true tissues
• Phylum Cnidaria
– Is one of the oldest groups in this clade
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• Cnidarians
– Have diversified into a wide range of both
sessile and floating forms including jellies,
corals, and hydras
– But still exhibit a relatively simple diploblastic,
radial body plan
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• The basic body plan of a cnidarian
– Is a sac with a central digestive compartment,
the gastrovascular cavity
• A single opening
– Functions as both mouth and anus
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• There are two variations on this body plan
– The sessile polyp and the floating medusa
Polyp
Medusa
Mouth/anus
Tentacle
Gastrovascular
cavity
Gastrodermis
Mesoglea
Body
stalk
Epidermis
Tentacle
Mouth/anus
Figure 33.5
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• Cnidarians are carnivores that use tentacles to
capture prey
• The tentacles are armed with cnidocytes
– Unique cells that function in defense and the
capture of prey
Prey
Tentacle
“Trigger”
Discharge
Of thread
Nematocyst
Figure 33.6
Coiled thread
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Cnidocyte
The phylum Cnidaria is divided into four major classes
– Hydrozoa, Scyphozoa, Cubozoa, and
Anthozoa
(a) These colonial polyps are members of
class Hydrozoa.
(b) Many species of jellies (class
Scyphozoa), including the
species pictured here, are
bioluminescent. The largest
scyphozoans have tentacles
more than 100 m long
dangling from a bell-shaped
body up to 2 m in diameter.
Figure 33.7a–d
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(c) The sea wasp (Chironex
fleckeri) is a member of
class Cubozoa. Its poison,
which can subdue fish and
other large prey, is more
potent than cobra venom.
(d) Sea anemones and other
members of class Anthozoa
exist only as polyps.
Hydrozoans
• Most hydrozoans
– Alternate between polyp and medusa forms
2 Some of the colony’s
polyps, equipped with tentacles,
are specialized for feeding.
3 Other polyps, specialized
for reproduction, lack
tentacles and produce tiny
medusae by asexual budding.
4 The medusae
swim off, grow, and
reproduce sexually.
Reproductive
polyp
1 A colony of
interconnected
polyps (inset,
LM) results
from asexual
reproduction
by budding.
Feeding
polyp
Medusa
bud
MEIOSIS
Gonad
Medusa
SEXUAL
REPRODUCTION
Sperm
Egg
ASEXUAL
REPRODUCTION
(BUDDING)
Portion of
a colony
of polyps
FERTILIZATION
Zygote
Developing
polyp
Mature
polyp
Planula
(larva)
Figure 33.8
1 mm
6 The planula eventually settles
and develops into a new polyp.
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5 The zygote develops into a
solid ciliated larva called a planula.
Key
Haploid (n)
Diploid (2n)
Scyphozoans
• In the class Scyphozoa,
– Jellies (medusae) are the prevalent form of the
life cycle
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Cubozoans
• In the class Cubozoa, which includes box
jellies and sea wasps
– The medusa is box-shaped and has complex
eyes
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Anthozoans
• Class Anthozoa includes the corals and sea
anemones
– Which occur only as polyps
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Concept 33.3: Platyhelminthes
• The vast majority of animal species belong to the
clade Bilateria
– Which consists of animals with bilateral
symmetry and triploblastic development
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Flatworms
• Members of phylum Platyhelminthes
– Live in marine, freshwater, and damp terrestrial
habitats
– Are flattened dorsoventrally and have a
gastrovascular cavity
• Although flatworms undergo triploblastic
development
– They are acoelomates
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Flatworms are divided into four classes
Table 33.2
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Turbellarian
• Turbellarians
– Are nearly all free-living and mostly marine
Figure 33.9
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• The best-known turbellarians, commonly called
planarians
– Have light-sensitive eyespots and centralized
nerve nets
Pharynx. The mouth is at the
tip of a muscular pharynx that
extends from the animal’s
ventral side. Digestive juices
are spilled onto prey, and the
pharynx sucks small pieces of
food into the gastrovascular
cavity, where digestion continues.
Digestion is completed within
the cells lining the gastrovascular cavity, which has
three branches, each with
fine subbranches that provide an extensive surface area.
Undigested wastes
are egested
through the mouth.
Gastrovascular
cavity
Eyespots
Figure 33.10
Ganglia. Located at the anterior end
of the worm, near the main sources
of sensory input, is a pair of ganglia,
dense clusters of nerve cells.
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Ventral nerve cords. From
the ganglia, a pair of
ventral nerve cords runs
the length of the body.
Monogeneans and Trematode
• Monogeneans and trematodes
– Live as parasites in or on other animals
– Parasitize a wide range of hosts
• Most monogeneans are parasites of fish
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• Trematodes that parasitize humans spend part of
their lives in snail hosts
1 Mature flukes live in the blood vessels of the human
intestine. A female fluke fits into a groove running
the length of the larger male’s body, as shown in
the light micrograph at right.
Male
Female
1 mm
5 These larvae penetrate
the skin and blood
vessels of humans
working in irrigated
fields contaminated
with infected human
feces.
2 Blood flukes reproduce
sexually in the human host.
The fertilized eggs exit the
host in feces.
3 The eggs develop in
water into ciliated
larvae. These larvae
infect snails, the
intermediate hosts.
Figure 33.11
4 Asexual reproduction
within a snail results in
another type of motile
larva, which escapes from
the snail host.
Snail host
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Cestoda
• Tapeworms
– Are also parasitic and lack a digestive system
Proglottids with
reproductive structures
200 µm
Scolex
Figure 33.12
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Hooks
Sucker
Concept 33.4: Mollusca
• Phylum Mollusca
– Includes snails and slugs, oysters and clams, and
octopuses and squids
• Molluscs have a muscular foot, a visceral mass,
and a mantle
• Most molluscs are marine
– Though some inhabit fresh water and some are
terrestrial
• Molluscs are soft-bodied animals
– But most are protected by a hard shell
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• All molluscs have a similar body plan with three
main parts
– A muscular foot
– A visceral mass
– A mantle
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Mollusc anatomy
Nephridium. Excretory organs
called nephridia remove metabolic
wastes from the hemolymph.
Heart. Most molluscs have an open circulatory
system. The dorsally located heart pumps
circulatory fluid called hemolymph through arteries
into sinuses (body spaces). The organs of the
mollusc are thus continually bathed in hemolymph.
The long digestive tract is
coiled in the visceral mass.
Visceral mass
Coelom
Intestine
Gonads
Mantle
Mantle
cavity
Stomach
Shell
Radula
Anus
The nervous
system consists
of a nerve ring
around the
esophagus, from
which nerve
cords extend.
Gill
Foot
Nerve
cords
Esophagus
Figure 33.16
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Mouth
Mouth
Radula. The mouth
region in many
mollusc species
contains a rasp-like
feeding organ
called a radula. This
belt of backwardcurved teeth slides
back and forth,
scraping and
scooping like a
backhoe.
• Most molluscs have separate sexes
– With gonads located in the visceral mass
• The life cycle of many molluscs
– Includes a ciliated larval stage called a
trochophore
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There are four major classes of molluscs
Table 33.3
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Chitons
• Class Polyplacophora is composed of the
chitons
– Oval-shaped marine animals encased in an
armor of eight dorsal plates
Figure 33.17
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Gastropods
• About three-quarters of all living species of
molluscs belong to class Gastropoda
(a) A land snail
Figure 33.18a, b
(b) A sea slug. Nudibranchs, or sea slugs, lost their shell
during their evolution.
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• Most gastropods
– Are marine, but there are also many
freshwater and terrestrial species
– Possess a single, spiraled shell
• Slugs lack a shell
– Or have a reduced shell
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• The most distinctive characteristic of this class
– Is a developmental process known as torsion,
which causes the animal’s anus and mantle to
end up above its head
Stomach
Mantle
cavity
Anus
Mouth
Figure 33.19
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Intestine
Bivalves
• Molluscs of class Bivalvia
– Include many species of clams, oysters,
mussels, and scallops
– Have a shell divided into two halves
Figure 33.20
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Anatomy of a bivalve
Hinge area
Mantle
Gut
Coelom
Heart
Shell
Adductor
muscle
Mouth
Anus
Excurrent
siphon
Palp
Water
flow
Foot
Figure 33.21
Mantle
cavity
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Gill
Incurrent
siphon
Cephalopods
• Class Cephalopoda includes squids and
octopuses
– Carnivores with beak-like jaws surrounded by
tentacles of their modified foot
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• Many octopuses
– Creep along the sea floor in search of prey
– Ambush prey from hiding places
Figure 33.22a
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(a) Octopuses are considered among the
most intelligent invertebrates.
• Squids use their siphon
– To fire a jet of water, which allows them to
swim very quickly
Figure 33.22b
(b) Squids are speedy carnivores with
beaklike jaws and well-developed eyes.
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Concept 33.5: Annelida
• Annelids are segmented worms
– They have bodies composed of a series of
fused rings
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The phylum Annelida is divided into three classes
Table 33.4
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Oligochaetes
• Oligochaetes (class Oligochaeta)
– Are named for their relatively sparse chaetae,
or bristles made of chitin
– Include the earthworms and a variety of
aquatic species
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• Earthworms eat their way through the soil,
extracting nutrients as the soil moves through the
alimentary canal
– Which creates pores for water and air flow, and
root growth, making soils conducive to plant
growth
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Anatomy of an earthworm
Epidermis
Cuticle
Circular
muscle
Chaetae
Septum
Longitudinal
muscle
Dorsal
vessel
Anus
Intestine
Nerve
cords
Nephrostome
Pharynx
Ventral
vessel
Clitellum
Esophagus
Metanephridium
Crop
Giant Australian earthworm
Intestine
Gizzard
Mouth
Subpharyngeal
ganglion
Table 33.23
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Polychaetes
• Members of class Polychaeta
– Possess paddlelike parapodia that function as
gills and aid in locomotion
Parapodia
Figure 33.24
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Leeches
• Members of class Hirudinea
– Are blood-sucking parasites, such as leeches
Figure 33.25
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Concept 33.6: Nematoda
• Nematodes are nonsegmented pseudocoelomates
covered by a tough cuticle
• Nematodes, or roundworms
– Are found in most aquatic habitats, in the soil,
in moist tissues of plants, and in the body
fluids and tissues of animals
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• Some species of nematodes
– Are important parasites of plants and animals
Encysted juveniles
Figure 33.27
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Muscle tissue
50 µm
Concept 33.7: Arthropoda
• Arthropods are segmented coelomates that have
an exoskeleton and jointed appendages
• Two out of every three known species of animals
are arthropods
• Members of the phylum Arthropoda are found in
nearly all habitats of the biosphere
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General Characteristics of Arthropods
• The diversity and success of arthropods
– Are largely related to their
• segmentation
• exoskeleton
• jointed appendages
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• Early arthropods, such as trilobites
– Showed little variation from segment to
segment
Figure 33.28
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• As arthropods evolved
– The segments fused, and the appendages
became more specialized
Cephalothorax Abdomen
Antennae
(sensory
reception)
Head Thorax
Swimming
appendages
Walking legs
Pincer (defense) Mouthparts (feeding)
Figure 33.29
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• The body of an arthropod
– Is completely covered by the cuticle, an
exoskeleton made of chitin
• When an arthropod grows
– It molts its exoskeleton in a process called
ecdysis
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• Arthropods have an open circulatory system
– In which fluid called hemolymph is circulated
into the spaces surrounding the tissues and
organs
• A variety of organs specialized for gas
exchange
– Have evolved in arthropods
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Concept 33.8: Echinodermata and Chordata
• Chordates and echinoderms share characteristics
of deuterostomes
– Radial cleavage
– Development of the coelom from the
archenteron
– Formation of the mouth at the end of the
embryo opposite the blastopore
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Echinoderms
• Sea stars and most other echinoderms
– Are slow-moving or sessile marine animals
• A thin, bumpy or spiny skin
– Covers an endoskeleton of hard calcareous
plates
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• Unique to echinoderms is a water vascular system
– A network of hydraulic canals branching into
tube feet that function in locomotion, feeding,
and gas exchange
Central disk
Madreporite. Water can flow
in or out of the water vascular
system into the surrounding
water through the madreporite.
Ring
canal
Tube
feet
Radial canal. The water vascular
system consists of a ring canal in the
central disk and five radial canals,
each running in a groove down the
entire length of an arm.
Branching from each radial canal are hundreds of hollow, muscular tube
feet filled with fluid. Movement occurs by sequential contraction and
relaxation of the tube feet.
Figure 33.39 A Sea Star
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Brittle Stars
• Brittle stars have a distinct central disk
– And long, flexible arms
Figure 33.40b (b) A brittle star (class Ophiuroidea)
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Sea Urchins and Sand Dollars
• Sea urchins and sand dollars have no arms
– But they do have five rows of tube feet that
function in movement
Figure 33.40c
(c) A sea urchin
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Selected Animal Phyla
Table 33.7
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