Download Phylum Cnidaria

CHAPTER 13
Radiate Animals
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Phylum Cnidaria
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Over 9,000 species in the phylum Cnidaria
Equipped with specialized cells: cnidocytes
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Fossil specimens dated to over 700 million
years ago
Extant species
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Contain a specialized stinging organelle, the
nematocyst
Most common in shallow marine environments
Some freshwater
None are terrestrial
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Phylum Cnidaria
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Some ctenophores, molluscs and flatworms
eat hydroids and use the stinging
nematocysts for their own defense
Four classes of Cnidaria
 Hydrozoa
 Scyphozoa
 Cubozoa
 Anthozoa
A fifth class, Staurozoa, has been proposed
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No medusae in life cycle but polyp topped by
medusa-like region
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Phylum Cnidaria
Characteristics of Phylum Cnidaria
 All are aquatic and mostly marine
 Symmetry
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Radial or biradial
Two body types
Free-swimming medusae
 Sessile polyps
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Diploblastic
Epidermis and gastrodermis
 Mesoglea: extracellar matrix that lies
between ectodermis and gastrodermis
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Phylum Cnidaria
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Incomplete gut: gastrovascular cavity
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Tentacles usually encircle mouth or oral
region
Muscular contractions via epitheliomuscular
cells
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Extracellular digestion in gastrovascular cavity
Intracellular digestion in gastrodermal cells
Outer layer of longitudinal fibers and an inner
layer of circular fibers
Sense organs for balance (statocysts) and
photosensitivity (ocelli)
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Phylum Cnidaria
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Nerve net
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Asexual reproduction
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Budding in polyps
Some colonies exhibit polymorphism
Sexual reproduction
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Symmetrical and asymmetrical synapses
Diffuse conduction
Two nerve rings in hydrozoan medusae
By gametes in all medusae and some polyps
Monoecious or Dioecious;
Holoblastic indeterminate cleavage
No excretory or respiratory system
Acoelomate
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Phylum Cnidaria
Form and Function
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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
 Aboral end attached to substratum by pedal
disc
 Reproduce asexually by budding, fission, or
pedal laceration
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Phylum Cnidaria
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In colonial forms
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In class Hydrozoa
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Polyps may be specialized for feeding,
reproduction, or defense
feeding polyps (hydranths) distinguished from
reproductive polyps (gonangia) by absence of
tentacles in gonangia
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Phylum Cnidaria
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Medusa
Bell or umbrella-shaped
 Usually free-swimming
 Mouth directed downward
 Tentacles may extend down from rim of
umbrella
 Medusae equipped with statocysts and ocelli
 Integration of sensory information into motor
response
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Velum differentiates hydromedusae from
scyphomedusae
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Function of a nerve ring located at base of the bell
Shelf-like fold of tissue from the bottom of the bell that
extends into the bell
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Phylum Cnidaria
Life Cycles
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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
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Produce other polyps asexually
Polyps eventually produce a free-swimming
medusa by asexual reproduction
 Budding or strobilation
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Phylum Cnidaria
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Medusae
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Dioecious
Reproduce sexually
True jellyfish (class Scyphozoa)
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Most colonial hydroids
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Medusa is large and conspicuous
Polyps typically very small
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
Body Wall
 Cnidarian body
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Outer epidermis
Inner gastrodermis
Layers separated by mesoglea
Mesoglea
 Gelatinous (at least 95% water)
 Continuous in polyps, extending through body
and tentacles
 Supports body
 Thick in anthozoans, and scyphozoan medusae
 Thinner in hydromedusae
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Phylum Cnidaria
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In Hydra
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Epidermis contains epitheliomuscular,
interstitial, gland, sensory, and nerve cells
Cnidarian bodies extend contract, bend,
and pulse
No mesodermally derived muscle cells
 Have epitheliomuscular cells
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Form most of epidermis
Cover organism and function in muscle contraction
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Phylum Cnidaria
Cnidocytes
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Many cnidarians are effective predators
Possible due to presence of a unique cell
type, the cnidocyte
Cnidoctyes
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Located in invaginations of ectodermal cells and
some gastrodermal cells
Produces one of over 20 types of cnidae
One type of cnida is the nematocyst
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Phylum Cnidaria
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Nematocyts
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Tiny capsules made of chitin-like material and
containing a coiled filament
End of capsule is covered
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
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Mechanism of Nematocyst Discharge
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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 manof-war can seriously harm humans
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Phylum Cnidaria
Feeding and Digestion
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Polyps
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Typically carnivorous
Catch prey with tentacles and pass them to the
gastrovascular cavity
Gland cells discharge enzymes to begin
extracellular digestion
Intracellular digestion continues in the
gastrodermis
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Phylum Cnidaria
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Polyps in colonial hydrozoans
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Feeding and digestion in hydromedusae
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Similar to that seen in the polyps
Schyphomedusae
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Pass food into a common gastrovascular cavity
Equipped with extended mouth edge (manubrium)
Used in capturing and ingesting prey
Anthozoan polyps
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Carnivorous
Expand and stretch tentacles in search of prey
Some small anthozoans
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Feed on minute forms captured by ciliary currents
Corals supplement their nutrition with carbon
collected from algal symbionts
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Phylum Cnidaria
Nerve Net
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Nerve net of cnidarians one of the best
examples of diffuse nervous system
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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,
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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
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Nerve cells synapse with both slender
sensory cells and epitheliomuscular
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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
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Most marine and colonial with both polyp and
medusa forms
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Hydra is not typical
Colonial Obelia is more exemplary
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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
Individual zooids are attached to the
hydrocaulus
 Hydranths (gastrozooids) are feeding
polyps with circle of tentacles surrounding
mouth
 Hydranths may be thecate or athecate
 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
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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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Phylum Cnidaria
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Freshwater Medusae
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Craspedacusta
May have evolved from marine ancestors
 Found in Europe, the United States, and parts
of Canada
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Craspedacusta medusae
May reach a diameter of 20 mm
 Polyps are tiny (2 mm)
 Polyp employs three methods of asexual
reproduction
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Phylum Cnidaria
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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
 Aboral 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
 Organism
moved by tentacles, and
engulfed by the mouth
 Opening of mouth activated by
glutathione
 Inside gastrovascular cavity, gland cells
discharge enzymes
 Myofibrils in nutritive-muscular cells run
at right angles to the body axis, forms a
weak circular muscle layer
 Water enters the gastrovascular cavity
due to beating cilia
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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
Hydras have three types of cnidae
 Penetrants penetrate prey and inject
poison
 Volvents recoil and entangle prey
 Glutinants secrete an adhesive for
locomotion and attachment
 Sensory cells among epidermal cells bear
a flagellum for chemical and tactile stimuli
and synapse with nerve cells
 Epidermal nerve cells are generally
multipolar with both one-way and two-way
synapses.
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Phylum Cnidaria
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Hydras reproduce sexually and asexually
 Asexual reproduction
 Budding
 Most hydra are dioecious
 Temporary gonads appear in autumn,
stimulated by lower temperatures or
stagnation
 Eggs and sperm shed externally
 Zygotes undergo holoblastic cleavage
to form hollow blastula
 Cyst forms around embryo
 Encysted form endures the winter,
then young hydras hatch in the spring
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Phylum Cnidaria
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Other Hydrozoans
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Orders Siphonophora and Chondrophora form
polymorphic swimming or floating colonies
Contain several types of polyp individuals
 Dactylozooids are
 Fishing tentacles that sting prey and lift
them to feeding polyps
 Gonophores
 Sacs containing ovaries or testes.
 In Physalia, the float, pneumatophore
 Thought to have expanded from the original
larval polyp
Other hydrozoans secrete calcareous skeletons
resembling true corals and are the hydrocorals
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Phylum Cnidaria
Class Scyphozoa
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Most of the larger jellyfishes belong to this
class
Nearly all float in open sea
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Bells vary in shape and size
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One order is sessile, attached to seaweeds by a
stalk
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
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Between lappets is a equilibrium sense
organ
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Mouth located beneath the umbrella
Manubrium forms four oral arms
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Rhopalium
 Contains a hollow statocyst
Capture and ingest prey
Tentacles, manubrium, and often entire body
may have nematocysts
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Phylum Cnidaria
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The nervous system consists of a nerve net
Sexes are separate
 Fertilization is internal in the gastric pouch
of the female
 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
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Medusa form is dominant
Polyp is inconspicuous or unknown
Umbrella is square
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One or more tentacles extend from each corner
At base of each tentacle is a flat blade called
a pedalium
Umbrella edge turns inward to form a
velarium
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Increases swimming efficiency
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Phylum Cnidaria
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Strong swimmers
Feed mostly on fish in nearshore areas
Polyp stage is tiny
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New polyps bud laterally
Do not produce ephyrae but directly change into
medusae
The sea wasp
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Potentially fatal cubomedusan from Australia
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Phylum Cnidaria
Class Anthozoa
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Lack a medusa stage
All marine, in both deep and shallow water,
and vary in size
Three subclasses: Zoantharia,
Cerianthpatharia, and Alcyonaria
Zoantharia and Cerianthpatharia are
hexamerous; Alcyonaria are octomerous
Gastrovascular Cavity
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Large and partitioned by septa or mesenteries,
inward extensions of body wall
Septa may be coupled or paired
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Phylum Cnidaria
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The mesoglea is mesenchyme containing
ameboid cells
No special organs for respiration or
excretion
Sea Anemones
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Polyps larger and heavier than hydrozoan
polyps
Attach to shells, rocks, timber, etc. by pedal
discs
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Phylum Cnidaria
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Some burrow in mud or sand
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
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Gastrovascular cavity divided into six pairs of
primary septa or mesenteries
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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
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Equipped with nematocysts
May protrude through mouth to help secure prey
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Phylum Cnidaria
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When in danger, water rapidly expelled through
pores as the anemone contracts to a small size
Feeding behavior under chemical control
 Asparagine activates feeding causing tentacles to
bend toward the mouth
 Reduced glutathione induces swallowing
Longitudinal muscles of the epidermis
 Only occur in the tentacles and oral disc
Longitudinal muscles of the column
 Gastrodermal and located in the septa
Most anemones can glide slowly on pedal discs
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Some can swim with limited ability
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Phylum Cnidaria
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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
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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
 Longitudinal and transverse fission, and
budding occur
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Phylum Cnidaria
Hexacorallian Corals
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Members of the order Scleractinia
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
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Tube Anemones and Thorny Corals
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Members of subclass Ceriantipatharia
Have coupled but unpaired septa.
Tube anemones
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Thorny or black corals
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Solitary and live in soft sediments
Colonial and attach to firm substrata
Both groups have few species and live in
warmer seas
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Phylum Cnidaria
Alcyonarian (Octocorallian) Corals
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Octomerous symmetry,
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All are colonial
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Eight pinnate tentacles
Eight unpaired complete septa
Gastrovascular cavities communicate through
tubes called solenia
Solenia pass through an extensive mesoglea
Show great variation in form of colony
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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
Hermatypic 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 live in their tissues
 Provide food for corals and recycle phosphorus
and nitrogenous wastes
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

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Phylum Cnidaria

Types of Reefs

Fringing Reef
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
Barrier Reef
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
Near the land with no lagoon or a very narrow
lagoon
Parallel to shore with a wide and deep lagoon
Atolls
Encircle a lagoon but not an island
 Have a steep bank on the seaward slope


Patch or Bank Reefs
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Some distance back from any steep slopes
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Phylum Cnidaria

Reef front or fore reef slope


Reef crest





Side facing the sea
Shallow water or emergent at top of the reef front
Wave action breaks pieces off
Reef flat toward the shore receives debris and
coralline sand
Support a diversity of corals and fish
Few nutrients enter or leave the system

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Little is lost efficiency in recycling among
interacting organisms
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Phylum Cnidaria




Nutrients from fertilizer and sewage threaten
coral reefs with excessive algal growth
Persian Gulf reefs have withstood surprising
amounts of oil pollution
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
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Phylum Cnidaria
Classification of Cnidaria





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
All marine, most prefer warm waters
Ctenophores



Eight rows of comb-like plates 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.
Oral-aboral axis, no head
Translucent body with a gelatinous layer that
contains muscle fibers (fibers are radial,
meridional, and latitudinal bands)
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Phylum Ctenophora



Complete gut
Acoelomate
Feeding Habits
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

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Trailing tentacles capture planktonic organisms
by means of epidermal glue cells called
colloblasts
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
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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
Eight equally-spaced bands called comb
rows extend from aboral to oral pole
Each band made of transverse plates of long
fused cilia called comb plates
Beat in each row begins at the aboral end
and moves along combs to the oral end
All eight rows beat in unison
Drives the animal forward mouth-first
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Phylum Ctenophora

Tentacles





Two tentacles are long, solid and extensible
Retract into a pair of tentacle sheaths
Surface bears colloblasts or glue cells that
secrete sticky material to hold animals
Body wall resembles cnidarians with a
gelatinous collenchyme in the interior
Muscle cells are distinct and not part of the
epitheliomuscular cells
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Phylum Ctenophora

Digestion and Feeding
Gastrovascular system comprises a
mouth, pharynx, stomach and canals that
run to the comb plates, tentacular sheaths,
and elsewhere
 Two blind canals terminate near mouth
 Aboral canal divides into two small anal
canals that expel wastes
 Digestion both extracellular and
intracellular

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Phylum Ctenophora


Respiration and excretion occur through
body surface
Nervous and Sensory Systems
 Resembles cnidarians
 Subepidermal plexus concentrated under
each comb plate
 Statocyst is a bell-like chamber


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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

When ctenophore contacts an unfavorable
stimulus



Cilia reverse their beat
Moves organisms backward
Comb plates are sensitive to touch

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Withdraw into the animal when touched
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Phylum Ctenophora

Reproduction and Development
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

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Monoecious
Fertilized eggs discharged through epidermis into
water
Cleavage is determinate
Free-swimming cydippid larva somewhat
resembles adult
Some consider cellular nature of mesoglea to
constitute a mesoderm
If mesoderm is derived from endoderm, both
ctenophores and cnidarians are diploblastic
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Phylum Ctenophora
Other Ctenophores

Beroe


Venus’ girdle


Band-like ctenophore over one meter long
Ctenoplana


Large conical ctenophore that lacks tentacles
flattened ctenophores that creep rather than swim
Most ctenophores are bioluminescent at night
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Phylogeny and Adaptive Diversification

Phylogeny of the Diploblasts

Distinctions between diploblastic and tripoblastic
conditions blurred


Ctenophores and cnidarians have typical
diploblastic characteristics


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Due to recent detailed morphological studies and
studies in gene expressions
Gelatinous middle surrounded by outer epidermal layer
derived from ectoderm
Inner gut lining derived from endoderm
However, the cells of the gelatinous layer are
problematic
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Phylogeny and Adaptive Diversification

If derived from endoderm




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Organisms are characteristic of triploblasts
But these cells may be derived from ectoderm
Some refer to this layer as ectomesoderm
Most cnidarians have few cells with the mesoglea,
so little debate whether they are diploblastic
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Phylogeny and Adaptive Diversification

However, during hydrozoan medusae stage





Development of the entocodon layer has led to
the suggestion that cnidarians are triploblastic
One product of the entocodon is striated muscle
Unlike contractile epitheliomuscular cells of other
cnidarians
Smooth and striated muscles are true muscle
cells
In triploblasts, true muscles are produced by
mesodermal cells


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Hydrozoans entocodon is ectodermal in origin
Other smooth muscle cells present in hydrozoan
medusae are of ectodermal origin
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Phylogeny and Adaptive Diversification
Probes of gene expression show that
homologous gene expression is occurring
in those of triploblast mesoderm and
diploblast mesoderm.
 Intepretation of these probes is not
complete


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May represent independent origin of muscle in
one branch of the diploblastic lineage
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Phylogeny and Adaptive Diversification


Recent re-examination of the development of
ctenophores has led to the observation that
muscle cells in the middle layer originate
from endodermal cells
If this is confirmed

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Ctenophores are triploblastic along with
bilaterally symmetrical animals.
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Phylogeny and Adaptive Diversification


Body symmetry also debated
Although adult ctenophore is radially
symmetrical



Cnidarian planula larva swims with one end
moving forward
This end could be designated as anterior end
giving the planula a distinct anterior-posterior
axis
The question remains: Did the radially
symmetrical cnidarians have a bilaterally
symmetrical ancestor or does the genetic
potential for bilateral symmetry predate the
bilateral body plan?
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Phylogeny and Adaptive Diversification

Given the yet unanswered questions to
origins of development


Branching order for the diploblastic phyla
is not yet determined
We predict a polytomy for cnidarian,
ctenophoran, and placozoan branches
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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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Phylogeny and Adaptive Diversification

Ctenophores
Adhere to the arrangement of comb plates and
biradial symmetry
 Vary in body shape and presence or absence
of tentacles

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Phylogeny and Adaptive Diversification

Cnidarian Phylogeny



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Relationships among cnidarian classes are still
controversial:
Which came first, the polyp or the medusa?
One hypothesis postulates that the basal
cnidarian was a trachyline-like hydrozoan with a
medusa stage.
Another hypothesis suggests that the basal
cnidarian was an anthozoan polyp without a
medusa in the life cycle.