Chapter 7
RADIATE ANIMALS CNIDARIANS
AND CTENOPHORES
7-1
Phylum Cnidaria
• Over 9,000 species in the phylum Cnidaria
• Equipped with specialized cells: cnidocytes
– Contain a specialized stinging organelle, the nematocyst
• Fossil specimens dated to over 700 million years ago
Species
– Most common in shallow marine environments
– Some freshwater
– None are terrestrial
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Phylum Cnidaria
Characteristics of Phylum Cnidaria
• All are aquatic and mostly marine
• Symmetry
– Radial or biradial
• Two body types
– Free-swimming medusae
– Sessile polyps
• Diploblastic
– Epidermis and gastrodermis
– Mesoglea: extracellar matrix that lies between
ectodermis and gastrodermis
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Phylum Cnidaria
• Four classes of Cnidaria
– Hydrozoa-hydroids,Portuguese man of war
– Scyphozoa-jellyfish
– Cubozoa-cube jellyfish
– Anthozoa-sea anemone and coral
• A fifth class, Staurozoa,stalked jellyfish, has been
proposed
– No medusae in life cycle but polyp topped by medusa-like
region
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Ecological Relationship
• Some ctenophores, molluscs and flatworms
eat hydroids and use the stinging nematocysts
for their own defense
• Some molluscs and fish feed on cnidarians
• Symbiotic relationship with crabs
• Coral reefs
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Figure 7_02a
Phylum Cnidaria
Form and Function
• Cnidaria have two basic body plans: polyp and
medusa
– Polyp
• Hydroid form
• Adaptation to a sedentary life
• Tubular body with the mouth directed upward and
surrounded by tentacles
• Mouth leads into a blind gastrovascular cavity
• Attach to substratum by pedal disc
• Reproduce asexually by budding, fission, or pedal
laceration
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Phylum Cnidaria
– In colonial forms
• Polyps may be specialized for feeding, reproduction, or
defense
– In class Hydrozoa
• feeding polyps (hydranths), and defense (dactylozooids)
distinguished from reproductive polyps (gonangia) by
absence of tentacles in gonangia
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Figure 7_02b
– Medusa
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•
•
•
Phylum Cnidaria
Bell or umbrella-shaped
Usually free-swimming
Mouth directed downward
Tentacles may extend down from rim of umbrella
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Phylum Cnidaria
• Colonial polyps are sessile.
• Hydra can move freely across a substrate by
gliding on their pedal disc aided by mucous
secretions.
• Sea anemone can move similarly on pedal
discs
• Medusae-contract the bell which expels water
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Phylum Cnidaria
Life Cycles
• Polyps and medusae play different roles in the
cnidarian life cycle
• Typically, zygote develops into a motile planula larva
• Planula settles, and metamorphoses into a polyp
– Produce other polyps asexually
– Polyps eventually produce a free-swimming medusa by
asexual reproduction
• Budding or strobilation
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Phylum Cnidaria
• Medusae
– Dioecious-separate sexes in each individual
– Reproduce sexually
– True jellyfish (class Scyphozoa)
• Medusa is large and conspicuous
• Polyps typically very small
– Most colonial hydroids
• Feature a polyp stage and a pelagic medusa stage
– Some hydrozoans (Physalia) form floating colonies
– In Hydra, only stage is a small freshwater polyp
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Phylum Cnidaria
Feeding and Digestion
Gastrovascular cavity acts as stomach.
– Catch prey with tentacles and pass them to the
gastrovascular cavity
– Gland cells discharge enzymes to begin extracellular
digestion
• Intracellular digestion continues in the cells of the
gastrodermis
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Phylum Cnidaria
Body Wall
• Cnidarian body
–
–
–
–
Outer epidermis
Inner gastrodermis
Layers separated by mesoglea
Mesoglea
• Gelatinous
• Continuous in polyps, extending through body and
tentacles
• Supports body
• Thicker in medusae
• Thinner in hydromedusae
• Gastrovascular cavity- Water enters due to
beating cilia
– Water serves as a hydrostatic skeleton
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Phylum Cnidaria
– Epitheliomuscular cells
• Form most of epidermis and cause muscular
contraction
– Undifferentiated interstitial cells
Develop into cnidoblasts, sex cells, buds, or
nerve cells, but not epitheliomuscular cells.
– Gland cells
• On the adhesive disc secrete an adhesive and
sometimes a gas bubble for floating
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Phylum Cnidaria
– In Hydra
• Epidermis contains epitheliomuscular, interstitial,
gland, cnidocytes, sensory, and nerve cells
– Cnidarian bodies extend contract, bend, and pulse
• No mesodermally derived muscle cells
• Have epitheliomuscular cells
– Form most of epidermis
– Cover organism and function in muscle contraction
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Phylum Cnidaria
Cnidocytes
• Many cnidarians are effective predators
• Possible due to presence of a unique cell type, the
cnidocyte
• Cnidoctyes
– Located in invaginations of ectodermal cells and some
gastrodermal cells
– Produces one of over 20 types of cnidae
– One type of cnida (tiny capsule) is the nematocyst
– End of cnida is covered by a little lid-operculum
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Phylum Cnidaria
• Nematocysts
– Tiny capsules made of chitin-like material and containing a
coiled filament
– Filament may have tiny barbs or spines
– Except in Anthozoa, cnidocytes equipped with trigger-like
cnidocil (modified cilium)
– Tactile stimulation cause nematocyst to discharge
– After Cnidae discharge, cnidocyte is absorbed and another
develops
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Phylum Cnidaria
• Mechanism of Nematocyst Discharge
– When stimulated, water to rush into the capsule
– The operculum opens and rapidly launches the
filament
– Barbs inject poison into prey
– Only a few jellyfish and the Portuguese man-of-war
can seriously harm humans
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Phylum Cnidaria
Nerve Net
• Nerve net of cnidarians one of the best examples of
diffuse nervous system
– Two nerve nets, one at the base of epidermis and one at
the base of gastrodermis, interconnect
• Nerve action potentials transmitted across synapses
by neurotransmitters
• Unlike higher animals,
– Nerve nets have neurotransmitters on both sides of the
synapses
– Allowing transmission in either direction
– No myelin sheath on axons
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Phylum Cnidaria
• Nerve cells synapse with both slender sensory cells
and epitheliomuscular
– Association often called a neuromuscular system
• The nerve net pattern is also found in annelid and
human (nerve plexus) digestive systems
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Phylum Cnidaria
Class Hydrozoa
• Most marine and colonial with both polyp and
medusa forms
– Freshwater Hydra is not typical
• Colonial Obelia is more exemplary
– Typical hydroid has a base, a stalk, and one or more
terminal zooids (individual polyp animals)
– Base is a rootlike stolon, or hydrorhiza
• Gives rise to stalks called hydrocauli
– Living part of the hydrocaulus is a tubular coenosarc
– Hydrocaulus covered by a non-living chitinous sheath, the
perisarc
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Phylum Cnidaria
• Hydra
– Found on the underside of aquatic leaves and lily
pads in clean fresh water
– Worldwide distribution (16 species in North
America)
– Body is a cylindrical tube
– Bottom end has a basal or pedal disc for
attachment
– The mouth (oral end) on a conical elevation, the
hypostome
– Ring of 6–10 hollow tentacles encircles mouth
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Phylum Cnidaria
– The mouth opens to a gastrovascular cavity
– Buds may project from the side, each develop a
mouth and tentacles
– Hydras feed on a variety of small crustaceans,
insect larvae, and worms
• Mouth is located on a raised hypostome, and
opens into the gastrovascular cavity
• Food organisms brush against the tentacles are
captured by nematocysts
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Phylum Cnidaria
– Hydras reproduce sexually and asexually
• Asexual reproduction
–Budding
• Most hydra are dioecious-separate sexes
–Temporary gonads appear in autumn,
stimulated by lower temperatures or
stagnation
–Eggs and sperm shed externally
–Cyst forms around embryo
–Encysted form endures the winter, then
young hydras hatch in the spring
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Phylum Cnidaria
– Hydroid Colonies
– Individual zooids are attached to the base
– Hydranths (gastrozooids) are feeding polyps with
circle of tentacles surrounding mouth
– Colonial hydroids bud off new individuals
• Individuals may be new hydranths or medusae
buds
– In Obelia, the medusae buds are formed by a
reproductive polyp called a gonangium
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Phylum Cnidaria
– Hydroid medusae
•
•
•
•
•
Usually smaller than schyphozoan medusae
Margin of the bell projects inward as a shelf-like velum
Mouth opens at the end of a suspended manubrium
Mouth connects to a stomach and four radial canals
Radial canals connect to a ring canal that runs around
the margin of the bell and connects with the hollow
tentacles
• Bell margin has many sensory cells
• Typically also bears statocysts, specialized sense organs
that function in equilibrium, and light-sensitive ocelli
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Hydroid-Gonionemus
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Phylum Cnidaria
• Other Hydrozoans
– Orders Siphonophora form floating colonies
– Contain several types of polyp individuals-feeding polyps,
reproductive polyps, nematocysts, jelly polyps
• In Physalia, the float, pneumatophore
– Thought to have expanded from the original larval
polyp
– tentacles can stretch up to 165 feet below the
surface of the water
– it is able to deflate itself and descend to escape the
threat
– Other hydrozoans secrete calcareous skeletons resembling
true corals and are the hydrocorals
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Phylum Cnidaria
Class Scyphozoa
• Most of the larger jellyfishes belong to this class
• Nearly all float in open sea
– One order is sessile, attached to seaweeds by a stalk
• Bells vary in shape and size
– Composed mostly of mesoglea
• Mesoglea contains ameboid cells and fibers
• Lack shelf-like velum found in hydrozoan medusae
• Margin of the umbrella has indentations, each
bearing a pair of lappets
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Phylum Cnidaria
• Between lappets is a equilibrium sense organ called a
rhopalium
• Mouth located beneath the umbrella
• Manubrium forms four oral arms
– Capture and ingest prey
• Tentacles, manubrium, and often entire body may
have nematocysts
• Feed on small organisms from protozoa to fish
• Cilia on gastrodermis circulates food and oxygen
throughout the animal
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Phylum Cnidaria
• Sexes are separate
– Fertilization is internal in the gastric pouch of the
female-sperm is carried to gastric pouches
– Zygote develops into a ciliated planula larva
• Attaches and develops into a scyphistoma
– Scyphistoma undergoes strobilation
– Form buds called ephyrae that break loose to form
jellyfish medusae
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Phylum Cnidaria
Class Staurozoa
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•
•
•
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
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Phylum Cnidaria
Class Cubozoa
• Medusa form is dominant
• Polyp is inconspicuous or unknown
• Umbrella is square
– One or more tentacles extend from each corner
• At base of each tentacle is a flat blade called a
pedalium
• Rhopalia has 6 eyes and other sense organs
• Umbrella edge turns inward to form a velarium
– Increases swimming efficiency
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Phylum Cnidaria
• Strong swimmers
• Feed mostly on fish in nearshore areas
• Polyp stage is tiny
– New polyps bud laterally
– Do not produce ephyrae but directly change into medusae
• The sea wasp
– Potentially fatal cubomedusan from Australia
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Phylum Cnidaria
Class Anthozoa-flower animals
• Lack a medusa stage
• All marine, in both deep and shallow water, and vary
in size
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Phylum Cnidaria
• Three subclasses:
• Zoantharia (aka Hexacorallia) -sea anemones
and hard coral
• Cerianthpatharia- black coral, tube anemone
• Octocorallia (aka Alcyonaria) –soft, flexible
and horny coral(sea fans)
• Zoantharia and Cerianthpatharia are
hexamerous(6); Alcyonaria are octomerous(8)
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Sea anemone-rose anemone
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Figure 7_21a
Figure 7_21b
7-59
Phylum Cnidaria
• Gastrovascular Cavity
– Large and partitioned by septa or mesenteries, inward
extensions of body wall
– Septa may be coupled or paired
• The mesoglea contains ameboid cells
• No special organs for respiration or excretion
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Phylum Cnidaria
Sea Anemones
• Polyps larger and heavier than hydrozoan polyps
• Attach to shells, rocks, timber, etc. by pedal discs Some
burrow in mud or sand
• Carnivorous
• Crown of tentacles surrounds the flat oral disc
• Slit-shaped mouth leads into a pharynx
• Siphonoglyph (ciliated groove) creates a water current
directed into the pharynx
• Transports oxygen, removes wastes, and maintains fluid
pressure for a hydrostatic skeleton
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Phylum Cnidaria
• Gastrovascular cavity divided into six pairs of primary
septa or mesenteries-tissues
– Smaller incomplete septa subdivide the large chambers
increasing surface area
– Free edge of each incomplete septum
• Forms a septal filament with nematocysts and gland
cells for digestion
• Acontia threads at lower ends of septal filaments
– Equipped with nematocysts
– May protrude through mouth to help secure prey
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Phylum Cnidaria
• When in danger, water rapidly expelled through pores as the
anemone contracts to a small size
• Most anemones can glide slowly on pedal discs
– Some can swim with limited ability
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Phylum Cnidaria
• When in danger, they contract and withdraw
tentacles and oral discs
• Most harbor symbiotic algae
• Some have a mutualistic relationship with hermit
crabs
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Phylum Cnidaria
• Reproduction
– Some dioecious, some monoecious
– Monoecious species are protandrous
• Produce sperm first and eggs later
– Gonads on margins of septa
– Fertilization is external or in gastrovascular cavity
– Zygote becomes a ciliated larva
– Pedal laceration, small pieces of pedal disc break off
and regenerate a small anemone
– Transverse fission, and budding occur
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Phylum Cnidaria
Zoantharian Corals
• Also called true or stony corals
• Described as miniature sea anemones that live in calcareous
cups they have secreted
• Gastrovascular cavity is hexamerous
– No siphonoglyph
• No pedal disc
– Secrete a limey skeletal cup with sclerosepta projecting up
into the polyp
• Sheet of living tissue forms over the coral surface
– Connects all gastrovascular cavities
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Phylum Cnidaria
Octocorallian Corals
Soft coral, sea pens, sea fans
• Octomerous symmetry,
– Eight tentacles
– Eight septa
• All are colonial
• Show great variation in form of colony
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Polyp of Octocorallian
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Octocorallian-soft coral
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Phylum Cnidaria
• Coral Reefs
• Great diversity of organisms
• Rivaled only by tropical rainforests
• Plants and animals are limited to top layer
– Above the calcium carbonate deposits
• Reef building corals and coralline algae form most coral reefs
– Require warmth, light, and salinity of undiluted sea water
– Limited to shallow waters between 30 degrees north and
30 degrees south latitude
– Photosynthetic zooxanthellae( red algae) live in their
tissues
– Provide food for corals and recycle phosphorus and
nitrogenous wastes
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Phylum Cnidaria
• Threatened Reefs
• Nutrients from fertilizer and sewage threaten coral
reefs with excessive algal growth
• Global warming coral tissues turn white and brittle,
this is called coral bleaching
• 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
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Phylum Cnidaria
Classification of Cnidaria
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•
•
•
•
Class Hydrozoa
Class Scyphozoa
Class Staurozoa
Class Cubozoa
Class Anthozoa
– Subclass Hexacorallia (Zoantharia)
– Subclass Ceriantipatharia
– Subclass Octocorrallia (Alcyonaria)
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Phylum Ctenophora
• Phylum composed of about 150 species-comb jellies
• All marine, most prefer warm waters
• Ctenophores
– Eight rows of comb-like plates of cilia used for locomotion
• Nearly all free-swimming, few creep or are sessile
• Body structure (ellipsoid or spherical shape)
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–
–
–
Biradial symmetry due to presence of two tentacles.
Tissue level of oganization
No head,
Translucent body with a gelatinous layer
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Phylum Ctenophora
• Feeding Habits
– 2 tentacles capture planktonic organisms by means of
epidermal glue cells called colloblasts-no nematocysts
– Short tentacles collect food on the ciliated body surface
– Ctenophores without tentacles feed on other gelatinous
animals
– Structuring classes within the Ctenophores still being
developed
– Luminesce
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Phylum Ctenophora
Representative Type: Pleurobrachia
• Transparent and 1.5–2 cm in diameter
• Oral pole bears the mouth opening
• Aboral pole equipped with statocyst-sense organ for
equilibrium and is associated with the beating of the
cilia
• Eight equally-spaced plates called comb plates or
ctenes extend from aboral to oral pole
• All eight rows beat in unison
• Drives the animal forward mouth-first
• Colloblast-stick cells on tentacles
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Phylum Ctenophora
• Digestion and Respiration
– Gastrovascular system comprises a mouth,
pharynx, stomach and canals that run to the comb
plates, tentacular sheaths, and elsewhere
– Digestion both extracellular and intracellular
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Phylum Ctenophora
• Respiration and excretion occur by diffusion across
the body surface
• Nervous and Sensory Systems
– Resembles cnidarians
– Statocyst is a bell-like chamber
• Tufts of cilia sense changes in pressure from statolith as
animal changes position
– Epidermis bears sensory cells sensitive to chemical
and other stimuli
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Phylum Ctenophora
• Reproduction and Development
– Monoecious
– Fertilized eggs discharged through epidermis into
water
– Free-swimming larva somewhat resembles adult
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Phylogeny and Adaptive Diversification
• Cnidarian Phylogeny
– Relationships among cnidarian classes are still
controversial:
Which came first, the polyp or the medusa?
– One hypothesis postulates that the ancestral cnidarian was
a trachyline-like hydrozoan with a medusa stage.
– Another hypothesis suggests that the ancestral cnidarian
was an anthozoan polyp without a medusa in the life cycle.
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Phylogeny and Adaptive Diversification
• Adaptive Diversification
– Neither phylum has deviated far from basic structural
plan
– Cnidarians
• Achieved large numbers of individuals and species
• Demonstrate large diversity considering the simplicity of body
plan
• Efficient predators
– Some feeding on prey larger than themselves
– Some feeding on small particles
• Some colonial forms grow to great size among corals
• Others show polymorphism and specialization of individuals
within a colony
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