Themes §  General characteristics of Cnidaria
ú  Why are they successful?
§  Cnidaria Diversity and Taxonomy
ú  Classes, SubClasses of Anthozoans
Phylum Cnidaria
§  Similarity of Polyp and Medusa
ú  The Cnidaria Body Plan
§  Class Scyphozoa, the jellyfish
More than 9, 000 species, including
jellyfish, corals, anemones, hydroids
and much much more.
What Characteristics Account for
the Great Success of Cnidarians?
1. Two distinct adult forms
2. Diphasic life cycle in some species
3. Ability to proliferate by cloning
4. And to form polymorphic colonies
5. Formidable prey capture device
6. Low Energy Demands
ú  Complexity, Sensory & Nervous System
What Characteristics Account for
the Great Success of Cnidarians?
1. Two Very Different Adult Forms
Polyp
Medusa
What Characteristics Account for
the Great Success of Cnidarians?
2. Dimorphic Life Cycle in Many Species
What Characteristics Account for
the Great Success of Cnidarians?
3. Ability to proliferate by cloning
Life cycle
Of the
Hydroid
Obelia
1
What Characteristics Account for
the Great Success of Cnidarians?
4. And to form polymorphic colonies
What Characteristics Account for
the Great Success of Cnidarians?
4. And to form polymorphic colonies
Physalia
Portuguese
Man-O-War
Hydractinea
What Characteristics Account for
the Great Success of Cnidarians?
5. Formidable prey-capture device
What Characteristics Account for
the Great Success of Cnidarians?
5. Formidable prey-capture device
Nematocyst
Prey tissue
Empty Cnidocyte
Different Toxins
§  Chironex fleckeri
§  T1 and T2
ú  Lethal myotoxins that
produce muscular
contractions
What Characteristics Account for
the Great Success of Cnidarians?
§  Physaliidae
6. Low Energy Demands
§  P3
ú  High molecular weight
toxins that reversibly
block glutamate
receptors which are
integral parts of the
nerve conductance
system
2
Diversity of Cnidarians
3 Main Taxonomic classes
Class
Class
Class
Scyphozoa
Anthozoa
Hydrozoa
Class Cubozoa contains
a few jellyfish spp
How is it that organisms as different
as jellyfish, hydroids and anemones
Can be classified in the same phylum?
Epitheleomuscular
cells
Nutritive
muscular
cells
Similarities of Polyp and Medusa
Nerve Nets and Muscular System
Mouth
Gland
cells
cnidocytes
mesoglea
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Epitheliomuscular Cells
and gastromuscular cells
Lacking mesoderm, but muscles derived
from epithelial endodermal and ectodermal cells
How is it that organisms as different
As jellyfish, hydroids and anemones
Can be classified in the same phylum?
1. Nematocysts: unique stinging cells
2. Diploblastic: 2 cell layers separated by acellular
mesoglea (“middle jelly”); epitheliomuscular cells
3. Primary radial symmetry
4. No head, no CNS, no circulatory or excretory
systems and no discrete respiratory structures
5.  Dimorphism of adult form; in life cycle
6.  2- way digestive system
7.  nerve net
Characteristics of the Class Scyphozoa
(Adaptations to a drifting, carnivorous lifestyle)
About
200
species
Characteristics of the Scyphozoa
mesoglea
gastric pouch
gonad
radial canal
stomach
gastrodermis
Radial
canal
mouth
Hydromedusa for
comparison
•  Small a few cm
•  Velum to aid in locomotion
•  Radial canals unbranched
•  Simple epidermal and endodermal nerve net
•  Nematocysts usually small without toxins
Characteristics of the Scyphozoa
(Adaptations to a drifting, carnivorous lifestyle)
Stomach
Stomach
G-V cavity more
complex, adapted
to subdue and
digest larger and
active prey
G-V Canals
MOUTH
Gastric Pouches
MANUBRIUM
4
Characteristics of the Scyphozoa
Characteristics of the Scyphozoa
(Adaptations to a drifting, carnivorous lifestyle)
mesoglea
gastric pouch
gonad
radial canal
stomach
gastrodermis
Radial
canal
mouth
G-V cavity more
complex, adapted
to subdue and
digest larger and
active prey
How are the
coronal muscles
connected to the
nervous system
that includes:
Coronal
Muscles
and
swimming
- Sensory neurons,
motor neurons
- Inner and outer nerve
rings
- Marginal ganglia
associated with
rhopalia that
constitute a pacemaker
How is swimming pace, direction coordinated?
Characteristics
of the Scyphozoa
To ganglia
Rhopalium
(Adaptations to a drifting,
carnivorous lifestyle)
The nervous system
is more complex;
unique sensory
structures
(than nerve net
shared by all
Cnidaria)
(Digestive)
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Rhopalia are Intergrated with Nerve Rings
24 eyes but no brain?
Garm et al., 2006
Cell Tissue Research #325
EN: epidermal nerve
GN: gastrodermal nerve
RC: Ring Canal
EN joins the nerve ring
Marginal ganglia
NCB: nerve cluster
Inner and outer nerve rings
RN: ring nerve
Coronal muscles
When did a Centralized Nervous System Evolve?
“Conventional” view
Nerve net
Epiheliomuscular system
Cnidarian nerve net e.g. Hydra
Marlow et al., 2009, Developmental Neurobiology
“Sub-functionalized” N.S.
Sea Anemone Nematostella
Nerve rings are Core of CNS in medusae
-- Concentrations of hundreds of axons in parallel form coupled nerve rings.
Having an annular form rather than a large ganglion does not make it
less centralized.
-- The annular configuration is a function of radial symmetry
Mackie, 2004. Neurosignals #13
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