5. 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.
6. 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.
7. 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.
8. Phylum Cnidaria
In colonial forms
that share a
gastrovascular
cavity, polyps may
be specialized for
feeding,
reproduction, or
defense.
9. 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.
12. 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.
13. 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.
15. 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).
16. 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.
18. Classification
A fifth class, Staurozoa, has been proposed.
No medusae in life cycle but polyp topped by
medusa-like region.
19. 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.
21. Class Hydrozoa – Typical Life Cycle
Most hydrozoans alternate between polyp and
medusa forms.
Some have only polyps.
Some have only medusae.
22. 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
23. 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
24.
25. Class Hydrozoa
Members of the order
Siphonophora, such as
the Portuguese man-of-
war, 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
27. 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.
28. 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
29. Class Scyphozoa – Typical Life
Cycle
Planula larvae
develop into a
polyp-like form.
Saucer-like buds
called ephyrae
are produced by
strobilation.
30. Class Cubozoa
In the class
Cubozoa, which
includes box jellies
and sea wasps, the
medusa is box-
shaped and has
complex eyes.
Polyps are tiny and
develop directly into
medusae.
http://youtu.be/CCuNMIT67y8
33. 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.
34. 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
35. 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.
36. Mutualisms
Sea anemones sometimes harbor zooxanthellae
(photosynthetic protists) like hard corals do.
Some crabs will decorate their shells with anemones.
37. 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.
40. Hexacorallian Corals
Polyps may be retracted into the skeleton.
Often retracted during the day.
The polyps expand for feeding.
41. Tube Anemones and Thorny Corals
Members of
subclass
Ceriantipatharia.
Have coupled but
unpaired septa.
Tube anemones
Solitary and live in
soft sediments.
42. 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.
45. Cnidaria
Medusozoa
Anthozoa Staurozoa Scyphozoa Cubozoa Hydrozoa
Trachyline-
like
hydrozoaGut with
septal
filaments
Siphonoglyph
Anthozoan
pharynx
Hexaradial
and
octaradial
symmetry
Creeping
planula
without cilia
Strobilation Complex eyes
Velarium
Boxlike
medusa body Polyp lost
Velum in medusae
Medusae produced by
lateral
budding and entocodonMedusoid body form
Motor nerve net
Primary polyp tentacles hollow
Mouth surrounded by solid tentacles
Planula larva
Cnidocytes
Radial, polypoid body form
Rhopalium
Polyp
reduced
or lost
Hydroids Man-of-war Other
hydrozoa
Phylum Cnidaria
46. 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.
47. Coral Reefs
The main structure of
the reef is calcium
carbonate secreted by
hermatypic (reef-
building) corals and
coralline algae.
53. 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.
54. 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
55. 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.
57. Phylum Ctenophora
They use their
ciliated comb
plates for
swimming.
Not strong
swimmers.
Ctenophores are
bioluminescent.
Statocyst – sense
organ
58. Phylogeny
Cnidarians may have evolved from a radially
symmetrical planula-like ancestor.
Trichocysts and toxicysts found in some
ciliates may be precursors to nematocysts.
