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Figure 33.21b
Chapter 33
Octopus
An Introduction to Invertebrates
Invertebrates are animals that lack a backbone.
(Trivia: they account for more than 95% of known
animal species.)
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Figure 33.3a
Porifera
(5,500 species)
Acoela
(400 species)
Ctenophora
(100 species)
1.5 mm
A sponge
Cnidaria
(10,000 species)
Acoel flatworms (LM)
Placozoa (1 species)
0.5 mm
A ctenophore, or comb jelly
A jelly
A placozoan (LM)
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Figure 33.3ba
Lophotrochozoa
A marine flatworm
Ectoprocta
(4,500 species)
Ectoprocts
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Rotifera
(1,800 species)
100 µm
Platyhelminthes
(20,000 species)
A rotifer (LM)
Brachiopoda
(335 species)
A brachiopod
Figure 33.3bb
Lophotrochozoa
Acanthocephala
(1,100 species)
Mollusca
(100,000 species)
Annelida
(16,500 species)
Curved
hooks
A marine
annelid
An octopus
An acanthocephalan (LM)
Nemertea (900 species)
Cycliophora (1 species)
100 µm
A cycliophoran (colorized SEM)
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A ribbon worm
Figure 33.3c
Ecdysozoa
Loricifera (10 species)
Priapula (16 species)
Tardigrada (800 species)
100 µm
50 µm
A loriciferan (LM)
A priapulan
Onychophora
(110 species)
Nematoda
(25,000 species)
Tardigrades (colorized SEM)
Arthropoda
(1,000,000 species)
A roundworm
A scorpion
(an arachnid)
An onychophoran
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Figure 33.3d
Deuterostomia
Hemichordata (85 species)
An acorn worm
Chordata
(56,000 species)
A tunicate
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Echinodermata
(7,000 species)
A sea urchin
Sponges are animals that lack true tissues
• Animals in the phylum Porifera are known informally as sponges
• They are sedentary and live in marine waters or fresh water
• Sponges are filter feeders, capturing food particles suspended
in the water that passes through their body
• Water is drawn through pores into a cavity called the
spongocoel and out through an opening called the osculum
• Sponges lack true tissues and organs although they do have
different cell types
• Choanocytes, flagellated collar cells, generate a water current
through the sponge and ingest suspended food
• Sponges consist of a gelatinous noncellular mesohyl layer
between two cell layers
• Wandering though the mesohyl are amoebocytes. They take
up food from water and from choanocytes, digest it, and carry
nutrients to other cells.
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●
●
●
●
Most sponges are hermaphrodites: Each individual functions
as both male and female producing gametes that are released
After fertilization, flagellated larva swim until they find a
suitable location, when they will attach and develop into a
sessile adult
Amoebocytes also secrete tough skeletal fibers within the
mesohyl. In some groups of sponges, these fibers are sharp
spicules of calcium carbonate or silica. Other sponges
produce more flexible fibers from a collagen protein called
spongin.
●
(We use these pliant, protein honeycombed skeletons as
bath sponges.)
Sponges are classified by their type of skeleton/spicules:
silica (glass sponges), calcium carbonate (chalk
sponges) , or spongin (bath sponges).
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Figure 33.4
Choanocytes
Collar
Food particles
in mucus
Choanocyte
Osculum
Spongocoel
Flagellum Phagocytosis
Amoebocyte
of food
particles
Pores
Spicules
Epidermis
Water
flow
Amoebocytes
Mesohyl
Azure vase sponge
(Callyspongia plicifera)
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Concept 33.2: Cnidarians are an ancient phylum
of eumetazoans
 All animals except sponges and a few other
groups belong to the clade Eumetazoa, animals
with true tissues
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Cnidarians
• Cnidarians have diversified into a wide range of both sessile and motile
forms including jellies, corals, and hydras
• They exhibit a relatively simple diploblastic, radial body plan
• The basic body plan of a cnidarian is a sac with a central digestive
compartment, the gastrovascular cavity
• A single opening functions as mouth and anus
• There are two variations on the body plan: the sessile polyp and motile
medusa
• A polyp adheres to the substrate by the aboral end of its body
• A medusa has a bell-shaped body with its mouth on the underside and
do not attach to the substate but move freely
• Cnidarians are carnivores that use tentacles to capture prey
• The tentacles are armed with cnidocytes, unique cells that function in
defense and capture of prey. Nematocysts are specialized organelles
within cnidocytes that eject a stinging thread
• Movements of contractile cells are controlled by a noncentralized nerve
net associated with simple sensory receptors that are distributed
radially around the body.
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Figure 33.5
Mouth/anus
Polyp
Tentacle
Medusa
Gastrovascular
cavity
Gastrodermis
Body
stalk
Mesoglea
Epidermis
Tentacle
Mouth/anus
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Figure 33.6
Tentacle
Cuticle
of prey
Thread
Nematocyst
“Trigger”
Thread
discharges
Cnidocyte
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Thread
(coiled)
Figure 33.7
The four classes of Phylum Cnidaria
(b) Scyphozoa
(a) Hydrozoa
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(c) Cubozoa
(d) Anthozoa
Phylum Cnidaria is divided into four major classes
• In the class Scyphozoa, jellies (medusae)
are the prevalent form of the life cycle
Jellies
• The class Cubozoa, which includes
box jellies, the medusa is box-shaped
and has complex eyes in the fringe of
the medusa. Cubozoans often have
highly toxic cnidocytes
• Class Anthozoa includes the corals and
sea anemones, and these cnidarians
occur only as polyps
•Corals secrete a hard external skeleton.
Each polyp generation builds on the
skeletons of earlier generations to form
the structures we call coral
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Sea anemones
Video: Clownfish and Anemone
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Figure 33.8-3
Most hydrozoans alternate between
polyp and medusa forms
Feeding
polyp
Reproductive
polyp
Medusa
bud
MEIOSIS
Gonad
Medusa
Egg
ASEXUAL
REPRODUCTION
(BUDDING)
Portion
of a
colony
of polyps
1 mm
SEXUAL
REPRODUCTION
FERTILIZATION
Developing
polyp
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Sperm
Zygote
Planula
(larva)
Mature
polyp
Hydra, a freshwater cnidarian, exists
only in polyp form and reproduces
asexually by budding
Key
Haploid (n)
Diploid (2n)
Lophotrochozoans
• Bilaterian animals have bilateral symmetry and
triploblastic development. Most have a coelom and a
digestive tract with two openings
• The clade Bilateria contains Lophotrochozoa,
Ecdysozoa, and Deuterostomia
• Lophotrochozoa includes the flatworms, rotifers,
ectoprocts, brachiopods, molluscs, and annelids
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Flatworms
 Members of phylum Platyhelminthes live in marine,
freshwater, and damp terrestrial habitats
 Although flatworms undergo triploblastic development,
they are acoelomates
 Flatworms have a gastrovascular cavity with one
opening
 Gas exchange takes place across the surface, and
protonephridia regulate the osmotic balance
 Their dorsoventrally flattened shape maximizes
surface area for gas exchange
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The best-known free-living flatworms are planarians
Planarians live in fresh water and prey on smaller animals
Planarians have light-sensitive
eyespots centralized nerve nets Pharynx
Gastrovascular
cavity
Mouth
Eyespots
Ventral nerve cords
Ganglia
Their nervous system is more complex and centralized than the
nerve nets of cnidarians
Planarians are hermaphrodites and can reproduce sexually, or
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Parasitic Species
 Parasitic flatworms live in or on other animals
 Two important groups of parasitic flatworms are the
trematodes and the tapeworms
 Trematodes parasitize a wide range of hosts, and most
have complex life cycles with alternating sexual and
asexual stages
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Figure 33.11
Trematodes that
parasitize
humans spend
part of their
lives in snail
Hosts.
They produce
surface proteins
that mimic
their host and
release
molecules
that manipulate
the host’s
immune
System.
Human
host
Female
1 mm
Mature flukes
Motile
larva
Ciliated
larva
Snail host
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Male
• Tapeworms are parasites of vertebrates and lack a digestive
system. (Tapeworms absorb nutrients from the host’s intestine.)
• Suckers and hooks on the head, or scolex, anchor the worm in
the digestive tract of the host.
• A long series of segments or proglottids, sacs of sex organs, lie
posterior to the scolex.
• Mature proglottids, loaded with thousands of eggs, are released
from the posterior end of the tapeworm and leave
with the host’s feces.
• Hermaphroditic and self-fertilizing, the proglottids
near the head are male and produce sperm.
Hooks
As more segments are added from the head end, Sucker
the existing segments become
Proglottids with
female and produce eggs which reproductive
structures
are fertilized by the sperm from
the anterior end.
Scolex
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Molluscs
 Phylum Mollusca includes snails and slugs, oysters and
clams, and octopuses and squids
 Most molluscs are marine, though some inhabit fresh water
and some snails and slugs are terrestrial
 Molluscs are soft-bodied animals, but most are protected
by a hard shell
 All molluscs have a similar body plan with three main
parts
 Muscular foot
 Visceral mass
 Mantle
 Many molluscs also have a water-filled mantle cavity
and feed using a rasplike radula
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Figure 33.15
Metanephridium
Visceral
mass
Coelom
Heart
Intestine
Gonads
Mantle
Digestive tract
Stomach
Shell
Mantle
cavity
Mouth
Radula
Anus
Gill
Foot
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Nerve
cords
Esophagus
Mouth
Radula
 Most molluscs have separate sexes with gonads located
in the visceral mass, but many snails are hermaphrodites
 The life cycle of many molluscs includes a ciliated larval
stage called a trochophore
 Four of the major classes of molluscs are
 Polyplacophora (chitons)
 Gastropoda (snails and slugs)
 Bivalvia (clams, oysters, and other bivalves)
 Cephalopoda (squids, octopuses, cuttlefish, and
chambered nautiluses)
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Figure 33.16
Chitons are oval-shaped marine animals encased in an
armor of eight dorsal plates
They use their foot like a suction cup to grip rock, and their
radula to scrape algae off the rock surface
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Figure 33.17
Gastropods: Snails and Slugs
About three-quarters of all living species of molluscs are gastropods
(a) A land snail
(b) A sea slug (nudibranch)
 Most gastropods are marine, but many are freshwater and terrestrial
 Gastropods move slowly by a rippling motion of the foot or by cilia
 Most have a single, spiraled shell that functions in protection from
injury, dehydration, and predation. Slugs lack a shell or have a
reduced shell.
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• A characteristic of gastropods is torsion, which causes the
animal’s anus and mantle to end up above its head. This
occurs during embryonic development, when the visceral
mass is rotated up to 180 degrees, so the anus and mantle
cavity are above the head in adults.
• After torsion, some of the organs that were bilateral are
reduced or lost on one side of the body.
• Torsion is different from the coiling of a shell.
• Many gastropods have distinct heads with eyes at the tips
of tentacles.
• Most gastropods use their scraping radula to graze on
algae or plant material.
• In place of the gills found in most aquatic gastropods, the
lining of the mantle cavity of terrestrial snails functions as a
lung.
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Bivalves
• Bivalves are marine and include many species of clams,
oysters, mussels, and scallops
• They have a shell divided into two halves drawn together
by adductor muscles
• Some bivalves have eyes and
sensory tentacles along the
edge of their mantle
• The mantle cavity of a bivalve
contains gills that are used for
feeding (they are suspension
feeders)as well as gas exchange
• Bivalves have no distinct head and the radula has been
lost.
• Motility varies-- sessile mussels attach themselves to rocks
by threads they secrete; clams can slowly pull themselves
into sand or mud using their muscular foot; scallops can
“swim” in short bursts by flapping their shells and jetting
water out of their mantle cavity
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Figure 33.19
Mantle
Hinge area
Coelom
Gut
Heart Adductor muscle
(one of two)
Digestive
gland
Anus
Mouth
Excurrent
siphon
Shell
Palp
Foot
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Mantle
cavity
Gonad
Gill
Water
flow
Incurrent
siphon
Cephalopods
 Cephalopods are carnivores with beak-like jaws
surrounded by tentacles (formed from the modified foot
which also forms the siphon)
 They are able to immobilize prey with a poison present in
their saliva
 Most octopuses creep along the sea floor in search of prey
• Cephalopods have a closed circulatory system, welldeveloped sense organs, and a complex brain
• The shell is reduced and internal in squids// lost in octopi
• Squids use their siphon to fire a jet of water, which allows
them to swim very quickly
• One small group of shelled cephalopods, the nautiluses,
survives today
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Figure 33.20
Squid
Octopus
Chambered
nautilus
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Figure 33.21
Molluscs are the animal group with the largest number of
recent extinctions
Other
invertebrates
Molluscs
An endangered
Pacific island
land snail,
Partula suturalis
Amphibians
Insects
Fishes
Birds
Mammals Reptiles (excluding birds)
Recorded extinctions of animal species
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Workers on a mound of pearl mussels killed to
make buttons (ca. 1919)
Annelids
 Annelids are coelomates with bodies composed of a
series of fused rings
 Recent molecular analyses indicate that the annelids
can be divided into two major clades, the Errantia and
the Sedentaria.
 Most members of clade Errantia are mobile, marine
organisms
 Many errantians have a pair of paddle-like or ridge-like
structures called parapodia on
each body segment. Each
parapodium has numerous
chaetae, bristles made of chitin
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Sedentarians
 Sedentarians tend to be less mobile than errantians.
Some species burrow into the substrate, while others live
in protective tubes
 Tube-dwelling sedentarians often have elaborate gills or
tentacles used for filter feeding
 This clade also contains the leeches and the earthworms
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Leeches
 Leeches include predators of
invertebrates, and parasites
that suck blood
 Leeches secrete a chemical called
hirudin to prevent blood from coagulating
 Most species of leeches live in
fresh water; some are marine or
terrestrial
Earthworms
 Earthworms eat through soil,
extracting nutrients as the soil
moves through the alimentary
canal
 Earthworms are hermaphrodites
but cross-fertilize
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Video: Earthworm Locomotion
Anus
Skin
Metanephridium
Clitellum
Esophagus
Crop
Pharynx
Intestine
Cerebral
ganglia
Gizzard
Mouth
Subpharyngeal
ganglion
Circulatory system vessels
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Ventral nerve cords
Concept 33.4: Ecdysozoans are the most
species-rich animal group
 Ecdysozoans are covered by a tough coat called a cuticle
 The cuticle is shed or molted through a process called
ecdysis
 The two largest phyla are nematodes and arthropods
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Nematodes
 Nematodes, or roundworms, are found in most aquatic
habitats, in the soil, in moist tissues of plants, and in body
fluids and tissues of animals
 They have an alimentary canal, but lack a circulatory
system
 Body wall muscles are all longitudinal, and their contraction
produces a thrashing motion
 Caenorhabditis elegans is a model
organism in research
 Some species of nematodes are
important parasites of plants and
animals. Trichinella spiralis can be acquired by humans
from undercooked pork
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Figure 33.27
Encysted juveniles
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Muscle tissue
50 µm
Arthropods
 Two out of every three known species of animals are
arthropods and they are found in nearly all habitats of the
biosphere
 The arthropod body plan consists of a segmented body,
hard exoskeleton called the cuticle made of layers of
protein and the polysaccharide chitin, and jointed
appendages
• The exoskeleton protects and provides an attachment site
for muscles. When an arthropod grows, it molts its
exoskeleton.
 The appendages of some living arthropods are modified for
functions such as walking, feeding, sensory reception,
reproduction, and defense. These modified appendages
are jointed and come in pairs
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 Arthropods show extensive cephalization with the number
of sense organs in the head, having eyes, olfactory
receptors, and antennae that function in touch and smell
 Arthropods have an open circulatory system in which
hemolymph is circulated into the spaces surrounding the
tissues and organs
 A variety of organs specialized for gas exchange have
evolved in arthropods
 Morphological and molecular evidence suggests that living
arthropods consist of three major lineages:
 Chelicerates (sea spiders, horseshoe crabs, scorpions,
ticks, mites, and spiders)
 Myriapods (centipedes and millipedes)
 Pancrustaceans (lobsters and other crustaceans, as
well as insects and their relatives)
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Chelicerates
• Chelicerates, subphylum Chelicerata, are named for
clawlike feeding appendages called chelicerae which
serve as pincers or fangs
• Two body parts- the cephalothorax and abdomen
• Lack sensory antennae and most have simple eyes
• Most marine cheliceriforms are extinct, but some species
survive today, including horseshoe crabs
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Figure 33.32
 Most
modern
chelicerates
are
arachnids,
which
include
spiders,
scorpions,
ticks, and
mites
50 µm
Scorpion
Dust mite
Web-building spider
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Arachnids:
• Arachnids have an abdomen and a cephalothorax, which
has six pairs of appendages: the chelicerae, the pedipalps
(which function in sensing or feeding), and four pairs of
walking legs
• Gas exchange in spiders occurs in respiratory organs
called book lungs
• Many spiders produce silk, a liquid protein, from
specialized abdominal glands
• The liquid is spun by spinnerets into fibers that solidify
• Web designs are characteristic of each species
• Silk is also used to wrap eggs or as “drop lines” for rapid
escapes
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Figure 33.33
Stomach
Intestine
Heart
Brain
Digestive
gland
Eyes
Ovary
Poison
gland
Anus
Book lung
Spinnerets
Silk gland
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Gonopore
(exit for eggs)
Sperm
receptacle
Chelicera
Pedipalp
Myriapods: millipedes and centipedes
All living myriapods are terrestrial
They have a pair of antennae and three pairs of appendages
modified as mouthparts
Millipedes eat decaying leaves
and plant matter. Millipedes
have many legs, with two
pairs per trunk segment
(a) Millipede
(b) Centipede
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Centipedes are
carnivores.
Centipedes have
one pair of legs per
trunk segment.
Poison claws on
the foremost trunk
segment paralyze
prey and aid in
defense
Crustaceans
 Crustaceans live in marine and freshwater environments
 Many crustaceans have highly specialized appendages
 Small crustaceans exchange gases through the cuticle;
larger crustaceans have gills
 Most crustaceans have separate males and females
 Isopods include terrestrial, freshwater, and marine species
 Pill bugs are a well-known group of terrestrial isopods
 Decapods are all relatively large crustaceans and include
lobsters, crabs, crayfish, and shrimp
 Planktonic crustaceans include many species of copepods,
which are among the most numerous of all animals
 They are rivaled in abundance by shrimplike krill
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Figure 33.35
Brine shrimp
A ghost crab, an example of a decapod
Pill bugs
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Barnacles
Insects
 Hexapoda is an enormous clade including insects and their
relatives
 Insects live in almost every terrestrial habitat and in fresh
water.
 Some insects are beneficial as pollinators, while others are
harmful as carriers of diseases, or pests of crops
 The internal anatomy of an insect includes several complex
organ systems
 The insect nervous system consists of a pair of ventral nerve
cords with several segmental ganglia. The two cords meet
in the head, where the ganglia are fused into a cerebral
ganglion (brain).
 In the complete digestive system, there are regionally
specialized organs with discrete functions.
 Most insects have separate males and females and
reproduce sexually
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Figure 33.38
Abdomen
Thorax Head
Compound eye
Antennae
Heart
Dorsal
artery
Crop
Cerebral ganglion
Anus
Vagina
Malpighian
tubules
Ovary
Tracheal tubes
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Nerve cords
Mouthparts
• Individuals find and recognize members of their own
species by bright colors, sound, or odors
 Flight is one key to the great success of insects
 One or two pairs of wings emerge from the dorsal side of
the thorax. Wings are extensions of the cuticle and are not
true appendages.
 Insect wings are very diverse: Dragonflies have two similar
pairs of wings. The wings of bees and wasps are hooked
together and move as a single pair. Butterfly wings work
together as the anterior wings overlap the posterior wings. In
beetles, the posterior wings function in flight, while the
anterior wings act as covers.
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 Many insects undergo metamorphosis during their
development
 In incomplete metamorphosis, the young, called nymphs,
resemble adults but are smaller and go through a series of
molts until they reach full size
 Some nymphs like grasshoppers look similar to the adult
while others like mayflies, look totally different
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Figure 33.40
 Insects with complete metamorphosis have larval stages
known by such names as maggot, grub, or caterpillar
 The larval stage looks entirely different from the adult stage
(a) Larva
(caterpillar)
(b) Pupa
(c) Later stage
(d) Emerging
pupa
adult
(e) Adult
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Concept 33.5: Echinoderms and chordates are
deuterostomes
 Echinoderms (phylum Echinodermata) include sea stars
and sea urchins
 Echinoderms and chordates constitute the clade
Deuterostomia
 Deuterostomes are defined primarily by DNA similarities
 Phylum Chordata consists of two basal groups of
invertebrates as well as vertebrates
 Chordates are bilaterally symmetrical coelomates with
segmented bodies
 Chordates did not evolve from echinoderms, but have
evolved separately from them for at least 500 million years
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Echinoderms
 Sea stars and most other echinoderms are slow-moving or
sessile marine animals
 A thin epidermis covers an endoskeleton of hard calcareous plates
 Echinoderms have a unique water vascular system, a network of
hydraulic canals branching into tube feet that function in
locomotion and feeding
 Males and females are usually separate, and sexual reproduction
is external
• Most adult echinoderms have radial symmetry with multiples of five
• Echinoderm larvae have bilateral symmetry
• Living echinoderms are divided into five classes: Asteroidea (sea
stars and sea daisies), Ophiuroidea (brittle stars), Echinoidea (sea
urchins and sand dollars), Crinoidea (sea lilies and feather stars),
and Holothuroidea (sea cucumbers)
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Asteroidea: Sea Stars
 Commonly known as starfish, Sea stars have multiple arms
radiating from a central disk
 The undersurface of each arm bears tube feet, which grip
substrate with adhesive chemicals
 Sea stars feed on bivalves by prying them open with their
tube feet, everting their stomach, and digesting their prey
externally with digestive enzymes
 Sea stars can regrow lost arms
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Digestive tract
Stomach
Anus
Spine
Gills
Madreporite
Central disk
Radial
nerve
Digestive glands
Ring
canal
Gonads
Ampulla
Podium
Tube feet
Radial canal
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Ophiuroidea: Brittle Stars
 Brittle stars have a distinct central disk and long, flexible
arms, which they use for movement
 Some species are suspension feeders, while others are
predators or scavengers
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Echinoidea: Sea Urchins and Sand Dollars
 Sea urchins and sand dollars have no arms but have five
rows of tube feet
 Their spines are used for locomotion and protection
 Sea urchins feed on seaweed using a jaw-like structure on
their underside
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Crinoidea: Sea Lilies and Feather Stars
 Sea lilies live attached to the substrate by a stalk
 Feather stars can crawl using long, flexible arms
 Both are suspension feeders
 Crinoidea have changed little over the course of evolution
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Holothuroidea: Sea Cucumbers
 Sea cucumbers lack spines, have a very reduced
endoskeleton, and do not look much like other
echinoderms
 Sea cucumbers
have five rows of
tube feet; some of
these are
developed as
feeding tentacles
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Figure 33.UN08
Phylum
Porifera (sponges)
Description
Lack true tissues; have choanocytes (collar
cells—flagellated cells that ingest bacteria and tiny
food particles)
Cnidaria (hydras, jellies, sea
anemones, corals)
Unique stinging structures (nematocytes) housed in
specialized cells (cnidocytes); diploblastic; radially
symmetrical; gastrovascular cavity (digestive
compartment with a single opening)
Dorsoventrally flattened acoelomates; gastrovascular
cavity or no digestive tract
Pseudocoelomates with alimentary canal (digestive
tube with mouth and anus); jaws (trophi); head with
ciliated crown
Coelomates with lophophores (feeding structures
bearing ciliated tentacles)
Platyhelminthes (flatworms)
Lophotrochozoa
Deuterostomia Ecdysozoa
Bilateria
Eumetazoa
Metazoa
Rotifera (rotifers)
Lophophorates: Ectoprocta,
Brachiopoda
Mollusca (clams, snails, squids)
Annelida (segmented worms)
Coelomates with three main body parts (muscular foot,
visceral mass, mantle); coelom reduced; most have
hard shell made of calcium carbonate
Coelomates with segmented body wall and internal
organs (except digestive tract, which is unsegmented)
Nematoda (roundworms)
Cylindrical pseudocoelomates with tapered ends;
no circulatory system; undergo ecdysis
Arthropoda (spiders, centipedes,
crustaceans, and
insects)
Coelomates with segmented body, jointed appendages,
and exoskeleton made of protein and chitin
Echinodermata (sea stars,
sea urchins)
Coelomates with bilaterally symmetrical larvae and
five-part body organization as adults; unique water
vascular system; endoskeleton
Coelomates with notochord; dorsal, hollow nerve cord;
pharyngeal slits; post-anal tail (see Chapter 34)
Chordata (lancelets, tunicates,
vertebrates)
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