Coelenterates are diploblastic animals with a radial or biradial symmetry and two tissue layers. They are divided into three phyla - Hydrozoa, Scyphozoa, and Anthozoa - based on their polyp and medusa forms. Hydrozoans have both polyp and medusa stages in their life cycle while scyphozoans are medusa-dominant and anthozoans are polyp-dominant. Cnidarians have stinging cells called nematocysts and a gastrovascular cavity that functions in digestion, circulation, and osmoregulation. They reproduce both sexually and asexually.

2. THE WORD COELENTERATE
MEANS “HOLLOW BODY CAVITY”.
History-
The term Phylum Coelenterata was
introduced by a German zoologist, Karl
Georg Friedrich (A)
Rudolf Leuckart (1822–1898) who split
Radiata into two phyla: Coelenterata and
Echinodermata.
Berthold Hatschek (1854–1941), split
Coelenterata into three phyla: Spongiaria,
Cnidaria and Ctenophora. (B)

3. Habit and Habitat
Almost 99% of the cnidarians are marine forms.
These are distributed throughout the world.
• Most of them are situated on the shore of the sea
and are littoral.
• Some of these marine forms are active swimmers
and are pelagic in habitat
• Only a few are found in deep sea. The deep sea
forms are often luminescent and emit light.
• Freshwater cnidarians are very few. The most
common example of fresh water cnidarians is
Hydra.

4. Body Organization
Cnidarians are diploblastic, radially symmetrical animals with
tissue level of body organization. In few cnidarians, symmetry is
biradial because of the elongation of mouth. Because of the
symmetry, Cnidaria were earlier kept under Kingdom Radiata,
along with ctenophores.
Largest: The largest noncolonial
Cnidarian is the Lion’s Mane Jellyfish,
Cyanea arctica*, (Bell-5.6 F diameter,
Tentacle legth-120 F) (A) & colonial
Cnidarian, Bubblegum Coral,
Paragorgia arborea (Hight 9.8 F, Its not a
coral)
Smallest: The smallest Cnidarian is the
tiny hydra, Psammohydra nanna,#
(height 0.02 inches).

5. Cnidarians are acoelomate
organisms. They do not have any
body cavity or any fluid-filled
space in their bodies. The body
wall of cnidarians consists of two
cellular layers (Diploblastic).
The outer epithelial layer is called
epidermis and the inner epithelial
layer is called gastrodermis. A
thin/thick, non-cellular jelly-like
layer, mesoglea is present in
between two cellular layers. The
water circulates through
gastrovascular cavity and acts as
hydrostatic skeleton.

6. The Polyp form is usually sessile
and has a cylindrical body with a
mouth and tentacles at the
upper free end.
Example: Coral & Hydra
The habitat of coelenterates is
salt water but some of them live
in fresh water.
They do not have a complete
nervous system but they have
nerve net .

7. *Coelenterates have a sac like gut
which serves as mouth and is
surrounded by tentacles.
*Their respiration method is
similar to that of sponges. They
respire only by means of diffusion
across the cell membrane.
*Coelenterates have two definite
tissue layers; an ectoderm and an
endoderm. Mesoglia is filled
between them

8. Most Coelenterates have stinging cells called nematocysts
on their tentacles.

9. Reproduction is Asexual and Sexual
Although most coelenterates reproduce sexually, some of them
reproduce by budding

10. Body form of Coelentarete: Hydra
The Polyp
form is
found in
hydra
mouth tentacle
endoderm
ectodermGastro vascular cavity
POLYP

11. All coelenterates contain a central
cavity with only one opening .
There are two general body forms
found among the coelenterates.
Polyp
Medusa

12. The structure of a hydra is
a typical example of a
coelenterate.
It consists of a body wall
within which is the gastro
vascular cavity.
A group of tentacles
surrounds the mouth that
opens into the cavity.

13. The body wall of
coelenterates has an
outer layer (ectoderm), an
inner layer (endoderm)
and an intermediate
layer, the mesoglia. The
intermediate layer
contains cells and jelly
like substances.

14. Body form of Coelenterates
tentacle
mouth
MEDUSA
mesoglea
endoderm
ectoderm
Gastro vascular cavity
The medusa
form is
commonly
seen in
jellyfish

15. The gastro vascular cavity serves
both as a circulatory and digestive
system.
It is surrounded by an inner layer of
the body that is made up of secretory
cells and columnar cells.
These cells have the ability to engulf
food particles. They are digested and
then diffuse form the body.

16. CNIDARIAN DIMORPHIC LIFE CYCLE

17. CLASSIFICATION OF PHYLUM
CNIDARIA
Phylum Cnidaria is divided into
three classes based on their
existence in different
morphological forms.
1-Hydrozoa
2-Scyphozoa
3-Anthozoa

18. Hyodrozoa Schyphozoa Anthozoa
Both Polyp & Medusa Medusa Dominant Polyp Dominant
Fresh water & Marine Marine Marine
Medusa neither cellular nor
fibrous (In Hydra Collagen and
Elastin Fiber Present)
Posses cell Posses cell and fiber
Gonads Ectodermal &
Discharge externally
Gonads Endodermal &
Discharge internally
Gonads Endodermal &
Discharge internally
Medusa Posses a Velum No Velum No Medusa
Tenticlocytes absent Tenticlocytes present in
Medusa
No Medusa
No Mesentries in Polyp No Polyp Mesentries prsent
Tentacles Hollow Solid Solid
Hydra, Obelia, Physalia Aurilia, Rhizostoma Metridium, Adamasia,
Gorgonia (Sea fen),
Pennatula (Sea Pen),
Corals

19. • These are solitary or colonial animals.
• These organisms have both polyp and medusa stages
in their life cycle.
• Medusae are craspedote with edge produced inwards
as true velum.
• Gastrovascular cavity present but without
stomodaeum and mesentries.
• Mesoglea is very thin.
A few examples are Hydra, Obelia, Millepora, Physalia
and Sertularia

20. (c) Pink Hearted
hydroids
(a) Physalia physalis, (b) Millepora

21. These are solitary and marine forms.
• Medusa stage is dominant in the life
cycle.
• Polyp stage may be absent or reduced
as a larval stage (scyphistoma).
• Medusae are acraspedote with
pseudovelum.
• Mesoglea is very thick and jelly-like.
A few examples are Aurelia, Pilema and
Periphylla

22. (a)Aurelia aurita
(b) Lion's Mane Jelly, a
venomous Scyphozoan

23. These are solitary or colonial animals which
exist in only polyp forms.
• Medusa stage is absent in the life cycle of
anthozoans.
• Gastrovascular cavity is well developed with
stomodaeum and mesentries beset with
nematocysts and gonads.
• They may be surrounded by calcareous
skeleton forming corals.
• Corals may reproduce by continous budding
and form large masses, coral reefs.
A few examples are Tubipora, Metridium,
Alcyonium and Gorgonia

24. Tubipora musica Metridium Tealia anemone

25. GLOSSARY
• Radial synmmetry: The symmetry in which an animal can be divided into two equal halves along
any plane passing through the axis
• Cnidocytes: The cells of organs and defence in cnidarians
• Epidermis: Outer cellular epithelial layer of the body wall
• Gastrodermis: Inner cellular epithelial layer of the body wall
• Gastrovascular cavity: Central gut cavity lined by endoderm which has a dual function of
digestion and transportation of food
• Hydropropulsion: Propulsion of the medusa in upward direction by forcing water out of the body
through alternate contraction and expansion of muscles
• Hypnotoxin: The proteinaceous poisonous fluid filled in the nematocysts which is injected into
the prey for its capture
• Manubrium: A central tubular projection in the body of medusa which bears a quandrangular
mouth
• Medusa: Umbrella-like, free-swimming and sexual morphological form of cnidarians
• Mesoglea: Thin/thick, non-cellular jelly-like layer present in between two cellular layers of body
wall
• Polyp: Tubular, sessile and asexual morphological form of cnidarians
• Tentaculocysts: Modified tentacles that serve as the organs of balance and equilibrium
• Totipotent cells: The undifferentiated embryonic cells which are capable of differentiating to any
other kind of cell required by an organism