CHAPTER 13
Radiate Animals
Phylum Cnidaria
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Phylum Cnidaria
Fossil specimens dated to over 700 million
years ago
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General Characteristics
Soft bodied
 Carnivorous
 Stinging tentacles
 Examples: Jellyfish, sea anemones
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Phylum Cnidaria
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Classes of Cnidaria
 Hydrozoa (Portuguese Man-of-War, Hydra)
 Scyphozoa (True Jellyfish - Aurelia)
 Anthozoa (Sea Anenome, Corals)
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Phylum Cnidaria
Characteristics of Phylum Cnidaria
 All are aquatic and mostly marine
 Symmetry
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Two body types
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Radial
Free-swimming medusa
Sessile polyps
Acoelomates - No coelom
Diploblastic
Blind (Incomplete) Gut
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Phylum Cnidaria
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Tentacles usually encircle mouth or oral region
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Mechanism of Nematocyst Discharge
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Equipped with specialized cells: cnidocytes
 Cnidocytes: stinging cells located on their tentacles
 Nematocyst: stinging organelle, poison-filled structure that
contains a tightly coiled dart.
 Filament may have tiny barbs or spines
 Tactile stimulation of cnidocil, causes nematocyst to discharge
 After discharge, cnidocyte is absorbed and another develops
When stimulated, water rushes into the capsule
The operculum (cap) opens and rapidly launches the filament
Barbs inject poison into prey
Only a few jellyfish - including the Portuguese man-of-war can
seriously harm humans
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Cnidarian Phyla
Topical
acidic
Solutions if stung:
(vinegar)
Urine - only if urine is acidic?? Most urine is sterile.
Apply heat or ice for pain (also denatures toxins)
Salt water should be applied to remove any excess toxins.
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Discharged
cnidae
recoiled
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Phylum Cnidaria
OTHER CNIDARIAN CHARACTERISTICS
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Sense organs for balance (statocysts) and
photosensitivity (ocelli)
Nerve net
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Asexual reproduction
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Budding in polyps
Sexual reproduction
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Simple nervous system, no brain
Interconnected Neurons
Used to sense physical contact
By gametes in all medusae and some polyps
Monoecious or Dioecious
No excretory or respiratory system
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A.
B.
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Hermit crab uses cnidrian as protection
Showing growth on host shell
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Phylum Cnidaria
Form and Function
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Cnidaria have two basic body plans:
polyp and medusa
 Polyp
Internal Body Cavity
 Adaptation to a sedentary life
 Tubular body with the mouth directed upward
and surrounded by tentacles
 Mouth leads into a blind-gut cavity
 Reproduce asexually by budding, fission, or
pedal laceration
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Phylum Cnidaria - Medusa
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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 (direction)
and ocelli (light/vision)
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Phylum Cnidaria
Body Wall
 Cnidarian body
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Outer epidermis
Inner gastrodermis (gastrovascular cavity = gut)
Layers separated by mesoglea
Mesoglea
 Gelatinous (at least 95% water) - jellylike
 Continuous, extending through body and
tentacles
 Supports body
 Thickness varies
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Phylum Cnidaria
Life Cycles
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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
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Hydrozoan Life Cycle
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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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Class Hydrozoa: Animals
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Hydra
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Portuguese Man-of-War
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Found in fresh water
Worldwide distribution (16 species in North America)
Body is a cylindrical tube
Ring of 6–10 hollow tentacles encircles mouth
Floats on surface (air bladder)
Tentacles reach up to 10 meters
Contains polyp colonies and medusa
Air bladder contains gases released
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Phylum Cnidaria
Class Hydrozoa
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Most marine and colonial with both polyp and
medusa forms (polyp form dominates)
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Hydra, hydrozoan, hydroid
Colonial Obelia
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Typical hydroid has a base, a stalk, and one or
more individual polyp animals
Feeding polyps with tentacles around mouth=
hydranths
In Obelia, the medusae buds are formed by a
reproductive polyp called a gonangium
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Obelia Life Cycle
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Hydroids
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Bell medusa
(Hydroid)
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Medusa
D. Developing Polyp from Planula: Frustule = bud
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Freshwater
Hydrozoan
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Hydra catching a water flea
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Hydra w/ developing bud and ovary/egg
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Portuguese man-of-war Colony
Secrete powerful neurotoxin - Class Hydrozoa (Many Polyps and medusa in colony)
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Differences between Hydrozoa and Scyphozoa
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Hydrozoa
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Colonial Stages
Mostly Polyp life form
Medusa, if present, are
usually small
Small amounts of mesoglea
Less tentacles
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Scyphozoa
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Large Medusa forms
Polyps are sedentary
Cells present in mesoglea
More complex
More tentacles
Large amount of mesoglea
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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
Bells vary in shape and size
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Composed mostly of mesoglea
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Class Scyphozoa
Giant jellyfish
-exceeds 2 Meters
in diameter
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Moon
Jellyfish
(Aurelia)
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Phylum Cnidaria - Scyphozoa
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Reproduction methods
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Asexual - Similar to other cnidarians. Budding
But also use Strobilation - asexual budding of
saucerlike buds that develop into medusa.
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Life Cycle of Aurelia, marine scyphozoan
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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
Examples: sea anemones, sea corals
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Sea Anemones
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Structure of Sea Anemone
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Phylum Cnidaria - Anthozoa
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 - Anthozoa
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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
Transports oxygen, removes wastes, and
maintains fluid pressure for a hydrostatic
skeleton
When in danger, water rapidly expelled through pores
as the anemone contracts to a small size
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Sea Anemone being attacked by a Sea Star
Can Sea Anemones run?
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Tube Anemone
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Phylum Cnidaria - Anthozoa
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Reproduction of Sea Anemones
Some dioecious, some monoecious
 Monoecious species - produce sperm first and
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eggs later
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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Closed during day, open to feed at night
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Polyp of
Coral
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A Thorny Coral
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A soft coral on Pacific Coral Reef
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Horny Coral Types
Red Gorgonian
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Sea Fan
Red Whip Coral
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Phylum Cnidaria
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Coral Reefs
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Great diversity of organisms (under rainforest)
Plants and animals are limited to top layer
 Above the calcium carbonate deposits
Reef forming corals and algae (precipitate calcium
carbonate) form most coral reefs
 Require warmth, light, and salinity of undiluted sea
water (contains calcium)
 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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Phylum Cnidaria - Human Impact
HUMAN IMPACT:
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Nutrients from fertilizer and sewage threaten coral
reefs with excessive algal growth - or by killing
zooxanthellae, which leads to coral bleaching.
Persian Gulf reefs have withstood surprising
amounts of oil pollution
Coral reefs in many areas are threatened by factors
mostly of human origin
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