Phylum Cnidaria
• A highly diverse assemblage that includes
jellyfish, sea anemonies, corals and
hydras.
Characteristics of Cnidaria
• Diplobastic Metazoa (Ectoderm and
Endoderm).
• Radialy Symmetrical.
• Specialized Cells Cnidocytes for Feeding.
• Two Body Types.
1) Polyp: Benthic.
2) Medusa: Pelagic.
CNIDARIA: HYDROZOA HYDRA SP. BUDDING
FROM NEBRASKA LINCOLN: POLYP FORM
HYDRA SP. TOP VIEW
UPSIDEDOWN JELLYFISH: MEDUSA FORM
Cassiopea frondosa
Tissue Layers
Diploblastic = 2 germ layers
• endoderm  gastrodermis
• ectoderm  epidermis
mesoglea
• gelatinous matrix between the 2 layers
epidermis
gastrovascular
cavity
mesoglea
gastrodermis
General Body Plan
• sac-like body (only 1 opening)
• Food and waste go in/out the same opening
 no anus!
• water within GVC acts as a hydrostatic skeleton
oral surface
gastrodermis
mesoglea
epidermis
aboral surface
mouth
Gastrovascular
cavity
General Life Cycle
HYDRA SP. FEEDING
A
B
C
D
Nematocysts
• specialized stinging organelles
• found within cnidocytes (cells)
• cnidocytes are located in epidermis
A cnidocyte
with a
nematocyst
within it
Nematocysts
• nematocysts are like “mini-harpoons”
• cnidocil senses movement & acts like a “trigger”
• can inject poison, coil around prey, or be adhesive
• functions:
- prey capture; defense
nematocyst
cnidocil
cnidocyte
Undischarged
Discharged
Nematocyst Types
HYDRA SP. UNDISCHARGED STENOTELE NEMATOCYSTS
DS
DS
HYDRA SP. UNDISCHARGED AND DISCHARGED
STENOTELE NEMATOCYSTS
A
S
HYDRA SP. UNDISCHARGED STENOTELE AND ATRICHOUS
ISORHIZA NEMATOCYSTS
A
S
HYDRA SP. DISCHARGED STENOTELE AND ATRICHOUS
ISORHIZA NEMATOCYSTS
Nematocyst Discharge
Biology
Feeding
– nematocysts within cnidocytes
– tentacles
Digestion
– extracellular (in GVC)
– intracellular (by gastrodermal cells)
– incomplete system (no anus)
Gas exchange & Excretion
– these systems are absent
Structure
Nervous System
– nerve net
(no central nervous system= no brain)
– sense organs
– statocysts (equilibrium organs)
– ocelli (photosensitive organs)
Skeletal System
– Mesoglea; water in GVC acts as a hydrostatic
skeleton, and some external skeletal system.
Reproduction
Reproduction
Asexual
– budding
– longitudinal fission
Reproduction
Reproduction
Sexual
– usually dioecious
(separate sexes e.g. humans)
– monoecious
(both male + female gonads in 1 individual)
– results in Planula larva
Phylum Cnidaria
Class Hydrozoa: Freshwater and
marine.
Class Scyphozoa: Jellyfish, all marine.
Class Anthozoa: Corals and
Anemones, all marine.
Class Cubozoa: Box jellyfish and Sea
Wasps, all marine.
Class Hydrozoa
• medusa & polyp body forms
Fire coral
Millepora
Class Hydrozoa
• medusa & polyp body forms
(medusa with velum)
• most are colonial
 colonies are formed of individual
zooids
a single zooid
Class Hydrozoa
• many of these colonies show polymorphism
 this is where there are several different types of
polyps/zooid and each type is specialized for a different
function
e.g. gastrozooids = feeding polyps
e.g. dactylzooids = defense polyps
 all the zooids within a colony are genetically identical
 these different zooids work together in the colony
Class Hydrozoa
- a sessile colony showing polymorphism
gonozooid
entire
colony
gastrozooid
Class Hydrozoa
- a Portuguese Man-o-war (Physalia) is a floating
hydrozoan colony showing polymorphism
Float
entire
colony
gastrozooid
dactylzooid
Portuguese Man-o-war Victims
Fire coral victim!
Class Hydrozoa- life cycle
sexual
reproduction
asexual
reproduction
Class Hydrozoa
• Hydra is an example of a solitary, freshwater hydrozoan
sexual
reproduction
gonads
asexual
No
Medusa
Stage
reproduction
bud
Class Hydrozoa
• Hydra is an example of a solitary, freshwater hydrozoan
asexual
reproduction
bud
Great Plains Hydrozoans
Chlorohydra viridissima: The Green Hydra
The green color arises from Chlorella-like unicellular
algae each occupying a vacuole in the endodermal cells.
Hydra vulgaris The common Hydra
Craspedacusta sowerbii
Velum
Lives predominantly as a polyp in small lakes,
ponds, and old water filled quarries the
medusa stage occurs during July-October.
Three ways of asexual reproduction
#1) Polyp budding.
#2) Non ciliated planulalike
buds may be produced.
Three ways of asexual reproduction
#3) Production of medusa buds.
Sexual
Reproduction
Asexual
Reproduction
Polypodium hydriforme
The most bizarre of the freshwater cnidarians and
one of the most unusual species of ALL
Cnidarians!!
Polypodium spends almost all of its
pre-adult life as a parasite of
Acipenseriform fishes.
LIFE CYCLE