Radiate Animals
Chapter 13
Radiate Animals
 Phylum Cnidaria & Phylum Ctenophora
 Radial Symmetry
 Diploblastic – 2 embryonic tissue layers
Phylum Cnidaria
 All animals except
sponges belong to
the clade
Eumetazoa, the
animals with true
tissues.
Phylum Cnidaria
 Phylum Cnidaria is
one of the oldest
groups in this clade.
 Fossil history back
700 MY.
Phylum Cnidaria
 Cnidarians have:




Radial symmetry
Gastrovascular cavity – extracellular digestion
Tissues – derived from two embryonic germ layers
Cnidocytes – special cells with stinging organelles
called nematocysts.
Phylum Cnidaria
 Cnidarians have
diversified into a wide
range of both sessile
and floating forms
including jellies,
corals, and hydras.
 Polymorphism –
some species exist
as both polyps and
medusae during their
life cycles.
Phylum Cnidaria
 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.
Phylum Cnidaria
 In colonial forms that
share a
gastrovascular
cavity, polyps may
be specialized for
feeding,
reproduction, or
defense.
Body Wall
 Cnidarians have an
outer tissue layer,
the epidermis,
derived from
ectoderm, and an
inner gastrodermis,
derived from
endoderm, with
jellylike mesoglea in
between.
Body Wall
 The epidermal layer contains several types of
cells organized into tissues.
Reproduction
 Polyps can reproduce asexually by budding,
fission, or pedal laceration.
Reproduction
 Cnidarians, typically medusae, can also reproduce
sexually.
 A zygote usually develops into a motile planula
larva.
 Some species only exist as polyps, others only as
medusae, others alternate between the two.
Feeding
 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.
 Nematocysts contain toxins used for prey capture and
defense.
Feeding
 Nematocysts are
used to inject a
toxin.
 Variable in form.
 May be used for prey
capture or defense.
Feeding
 Extracellular digestion begins in the
gastrovascular cavity, but is completed within
the cells of the gastrodermis.
 Some cnidarians supplement their diet with
nutrients collected from algal symbionts
(zooxanthellae).
Nerve Net
 Cnidarians have a diffuse nervous system.
 Nerve cells forming two interconnected nerve nets in
the epidermis and gastrodermis.
 No concentrated grouping of nerve cells forming a
central nervous system.
 CNS does not provide advantage for radially
symmetrical animals where stimuli approach from
all sides.
Classification
 The phylum Cnidaria is divided into four major
classes:
Class Hydrozoa Class Scyphozoa Class Cubozoa Class Anthozoa
Classification
 A fifth class, Staurozoa, has been proposed.
 No medusae in life cycle but polyp topped by
medusa-like region.
Phylum Cnidaria
 Class Staurozoa
 Commonly called stauromedusans
 No medusa stage.
 Solitary polyp body that is
stalked.
 Uses adhesive disk to attach to
seaweeds, and objects on sea
bottom.
 Polyp top resembles a medusa
with eight extensions (“arms”)
ending in tentacle clusters
surrounding mouth.
 Reproduce sexually.
Class Hydrozoa
 Hydrozoans are
variable in form.
 Mostly marine.
 Usually colonial,
sometimes solitary.
http://youtu.be/MpSPdeTN74I
Class Hydrozoa – Typical Life Cycle
 Most hydrozoans alternate between polyp and
medusa forms.
 Some have only polyps.
 Some have only medusae.
Class Hydrozoa - Polyps
 Typical hydroids have a
base, a stalk, and one or
more terminal zooids
(individual polyp
animals).
 Thecate – presence of a
protective cup around
the polyp.
 Obelia
 Athecate – no such
protection.
 Ectopleura
Class Hydrozoa - Medusae
 Hydroid medusae are usually
smaller than those in the class
Scyphozoa.
 Gastrovascular cavity is
continuous from mouth to
tentacles and is lined by
gastrodermis.
 Velum, inward projection of the
bell, is present.
 Specialized organs:
 Statocysts – equilibrium
 Ocelli – light sensitive
Class Hydrozoa
 Members of the order
Siphonophora, such as
the Portuguese man-ofwar, are actually
colonies of polyp
individuals.
 One polyp may be gas
filled and used as a
float.
 Feeding polyps each
with one long tentacle
 Reproductive polyps
Class Hydrozoa
 Hydrocorals
resemble true
corals.
 Calcareous
skeleton
 Fire coral
Class Scyphozoa
 In the class
Scyphozoa, jellies
(medusae) are the
prevalent form of the
life cycle.
 No velum present.
 Rhopalium – sense
organ containing
statocysts and
sometimes ocelli.
Class Scyphozoa
 Tentacles around
the periphery of the
bell contain
nematocysts used to
paralyze prey
animals.
 In the center are four
frilly oral arms used
to capture and
ingest prey.
http://youtu.be/aJUuotjE3u8
Class Scyphozoa – Typical Life
Cycle
 Planula larvae
develop into a
polyp-like form.
 Saucer-like buds
called ephyrae
are produced by
strobilation.
Class Cubozoa
 In the class
Cubozoa, which
includes box jellies
and sea wasps, the
medusa is boxshaped and has
complex eyes.
 Polyps are tiny and
develop directly into
medusae.
http://youtu.be/CCuNMIT67y8
Class Cubozoa
Class Anthozoa
 Class Anthozoa
includes the corals
and sea anemones
which occur only as
polyps – no medusa
stage.
 All marine
 Solitary or colonial
Sea Anemones – Order
Actinaria
 Found in coastal
waters all around
the world.
 Attach to rocks using
their pedal disc.
 Feed on fish or any
other food of
suitable size.
Sea Anemones – Order
Actinaria
 Sea anemones usually move by gliding slowly
along on their pedal discs.
 When a predator approaches, most withdraw.
 Stomphia detaches its disc and “swims” away.
http://youtu.be/Dm98n3908QM
Sea Anemones – Order
Actinaria
 Tentacles arranged around the central mouth.
 The gastrovascular cavity is divided into six
radial chambers.
 Increases the surface area of the gastrodermis.
Mutualisms
 Sea anemones sometimes harbor zooxanthellae
(photosynthetic protists) like hard corals do.
 Some crabs will decorate their shells with anemones.
Mutualisms
 Some damselfish (anemone fishes) form
associations with large anemones.
 Fish gains protection from living in the anemone.
 The fish may help ventilate the anemone, or
keep it free of sediment.
Hexacorallian Corals
 Hexacorallian
corals (Order
Scleractinia)
are the true or
stony corals.
 Like tiny
anemones
living in
calcareous
cups.
Hexacorallian Corals
 Hexacorallians
(Subclass
Hexacorallia) have
a gastrovascular
cavity subdivided by
septa in multiples of
six.
 The calcium
carbonate skeleton
is secreted below
living tissue.
Hexacorallian Corals
 Polyps may be retracted into the skeleton.
 Often retracted during the day.
 The polyps expand for feeding.
Tube Anemones and Thorny Corals
 Members of
subclass
Ceriantipatharia.
 Have coupled but
unpaired septa.
 Tube anemones
 Solitary and live in
soft sediments.
Tube Anemones and Thorny Corals
 Thorny or black corals
 Colonial and attach to firm substrata.
 Both groups have few species and live in
warmer seas.
Octocorallian Corals
 Members of the Subclass
Octocorallia have eight
pinnate tentacles and eight
septa.
Octocorallian Corals
 Sea fans, sea pens & sea pansies belong to
this group.
 Often beautifully colored.
Phylum Cnidaria
Cnidaria
Medusozoa
Anthozoa
Staurozoa
Gut with
septal
filaments
Scyphozoa
Strobilation
Complex eyes
Trachylinelike
hydrozoa
Hydroids
Man-of-war
Other
hydrozoa
Velarium
Boxlike
medusa body
Siphonoglyph
Anthozoan
pharynx
Hexaradial
and
octaradial
symmetry
Hydrozoa
Cubozoa
Creeping
planula
without cilia
Rhopalium
Polyp
reduced
or lost
Polyp lost
Velum in medusae
Medusae produced by
lateral
Medusoid body formbudding and entocodon
Motor nerve net
Primary polyp tentacles hollow
Mouth surrounded by solid tentacles
Planula larva
Cnidocytes
Radial, polypoid body form
Coral Reefs
 Coral reefs are one of the most productive and
diverse ecosystems on Earth.
 Found in shallow tropical seas.
 They require warm water.
 Sunlight required for symbiotic zooxanthellae.
Coral Reefs
 The main structure of
the reef is calcium
carbonate secreted by
hermatypic (reefbuilding) corals and
coralline algae.
Coral Reefs
 Octocorallian corals and some hydrozoans
(those known as fire coral) also contribute
calcium carbonate.
Types of Reefs
 Fringing reefs are close to a landmass with no
lagoon or a narrow lagoon.
Types of Reefs
 Barrier reefs run parallel to shore and have a
wide, deep lagoon.
Types of Reefs
 Atolls are reefs that encircle a lagoon, but not
an island.
Types of Reefs
 Patch reefs are scattered throughout a lagoon.
Reef Zones
 The fore reef slope, or reef front, is the side that
faces the sea.
 Slopes into deeper water.
 The reef crest is the shallow or even slightly emergent
top of the reef.
 The reef flat is the shallow back reef area that slopes
into the lagoon.
Coral Reefs
 Nutrients from fertilizer and
sewage threaten coral
reefs with excessive algal
growth.
 Coral reefs in many areas
are threatened by factors
mostly of human origin.
 Higher atmospheric concentrations of carbon dioxide
(from burning hydrocarbon fuels) tends to acidify
ocean water, which makes precipitation of CaCO3 by
corals more difficult metabolically.
http://www.youtube.com/watch?v=QAaRJKKTOys
Coral Reefs
 Symbiosis between
zooxanthellae and corals
is threatened by global
warming.
 Warmer water damages the
photosynthetic mechanism
in zooxanthellae.
 Coral tissues turn white and
brittle, this is called coral
bleaching.
 Zooxanthellae die or are
expelled by corals.
Phylum Ctenophora
 Phylum
Ctenophora are the
comb jellies.
 No nematocysts.
 Tissue level of
organization, like
cnidarians.
 Mostly free
swimming.
Phylum Ctenophora
 They use their
ciliated comb
plates for
swimming.
 Not strong
swimmers.
 Ctenophores are
bioluminescent.
 Statocyst – sense
organ
Phylogeny
 Cnidarians may have evolved from a radially
symmetrical planula-like ancestor.
 Trichocysts and toxicysts found in some ciliates
may be precursors to nematocysts.