Invertebrate zoology

1. LECTURE 6:PHYLUM CNIDARIA AND
PHYLUM CTENOPHORA
• Lecture outline
• Part 1-Phylum Cnidaria
• General Introduction
• The body wall and Nematocysts
• Alteration of Generations
• Maintenance Functions
• Reproduction in Cnidaria
• Classification and evolutionary
trends in Cnidaria
• Summary

2. 1. Introduction
• Cnidarians include jellyfish, sea anemones, corals,
hydroids and siphonophores (e.g. Portuguese man-of-
war)
• The Cnidarians are the first animals to show body
symmetry i.e. radial symmetry
• Have symmetry = body parts organized about a central
axis although they lack cephalization
• They are like the a bicycle spokes in wheel, with
multiple planes of symmetry.
• Radial symmetry allows animals cnidarians to respond
to stimuli from all directions.

3. • Cnidarians show a major evolutionary innovation as
compared with sponges e.g. food is digested outside
of cells in a gut cavity
• Also show the beginning of cell differentiation (true
tissues)
• Cnidarians are diverse in form as evidenced by their
ability to form colonies by asexual reproduction
• e.g siphonophores in the Portuguese Man of War)
• They also exhibit dimorphism-colonial or single polyps
(asexual and sessile), or medusae (sexual and
planktonic)
•
Cnidaria: General Introduction

4. • Polymorphism allows them to exploit different
environments and resources.
• Mostly marine, but some inhabit freshwater.
• They are suspension feeders or opportunistic
carnivores
• Some have symbiotic relationships with algae
(dinoflagellates) called zooxanthellae
• Size variable: individuals from mm up to 2 m across;
colonial forms such as corals may be much bigger
•
Cnidaria: General Introduction

5. • Like Ctenophorans, Cnidarians are diploblastic, i.e.
their body wall consists of two germ layers of cells:
ectoderm and endoderm.
• The endoderm develops into true tissue, including
tissue associated with the gut and associated glands.
• The ectoderm gives rise to the epidermis, the nervous
tissue, and if present, nephridia.
• Simpler animals, such as sea sponges, have one germ
layer and lack true tissue organisation.
• The complex animals (from flat worms to humans) are
triploblastic with three germ layers (a mesoderm,
ectoderm and endoderm).
• The mesoderm develop into true organs. Groups of
diploblastic animals alive today include jellyfish, corals,
sea anemones and comb jellies.
Distinguishing Characteristics of Phylum
Cnidaria

6. Distinguishing Characteristics of
Phylum Cnidaria
• Cnidarians exhibit a higher level of tissue organization
(have true tissues).
•The body plan demonstrates radial symmetry, oral
ends terminates in a mouth surrounded by tentacles
•Nervous system is composed of simple net with simple
sensory structures in some members.
•Cnidarians typically have a ciliated motile larva called
planula, and are mostly marine but some fresh water
forms exist.

7. Distinguishing Characteristics………
• Most cnidarians are carnivorous, trapping and killing
their prey by means of special stinging structures
called (cnidocysts)
• These tentacles (cnidocytes) bear stinging organelles
called nematocysts.
• Cnidocytes are specialized cells used
for defense and capturing prey.
• Nematocyst: stinging organelle in cnidocyte that has a
long filament coiled up inside it.
• Often form colonies of individuals specialized for
certain functions (polymorphism)
• e.g. feeding polyps, defensive polyps and reproductive polyps all
within the same colony.

8. Outer layer of cells –
the epidermis
Inner gastrodermis
which lines the gut
cavity or
gastrovascular cavity
2. Body Wall and Nematocysts
In between these tissue layers is a non-cellular jelly-
like material called mesoglea.
A sac-like body coelenteron serves as both mouth
and anus

9. 3. Maintenance and Function
• Polyps: hydrostatic support effected by water-filled
coelenteron
• It is constrained by circular and longitudinal muscles
• Colonial anthozoans may incorporate bits of sediment
and shell pieces into the column wall for further
protection.
•They have a flexible
chitinous perisarc
•In medusae the
mesoglea provide
support with jelly-like
substances
•It may sometimes have
a cartilaginous
consistency.

10. Feeding/Nutrition in Cnidarians
• All cnidarians are carnivores
feeding on live prey which they
usually capture using nematocyst-
laden tentacles.
• They capture small animals with
nematocysts and paralyze them
with poison injection.
• Tentacles push the prey into the
gastrovascular cavity through the
mouth.
• Digestion is partly extracellular and
partly intracellular.
• Extracellular digestion takes place
in the coelenteron through enzymes
secreted by gastrodermal cells
(gastrodermis) lining the cavity.

11. Functioning of Nematocysts
• After receiving a signal, the operculum is moved back
• This allows a hollow structure known as a tubule to fire.
• This tubule may contain toxins (as is the case with
nematocysts) adhesives that stick to the prey

12. Excretion, gaseous exchange and
sensitivity in Cnidarians
• Both the epidermis and the gastrodermis absorb
oxygen for respiration directly from the environment
or from the GV cavity.
• Oxygen then diffuses to any underlying cells.
• In the same manner waste products, such as
carbon dioxide and ammonia (= ammonotelic)
diffuse outward from cells to the body surface.
• There are no specialized organs or body surfaces
for gas exchange or the elimination of wastes.
• The nervous system is a plexus of neurons serving
sensory and motor systems (nerve net)

13. Nervous system
•The nerve net is the simplest form of a nervous system
found in multicellular organisms..
•Neurons found in nerve
nets are found spread
apart.
•This nervous system
allows cnidarians to
respond to physical
contact.
•They can detect food
and other chemicals in a
rudimentary way

14. Locomotion in Cnidaria
• Polyps are generally
sessile; medusae are
planktonic
• Polyps such as
anthozoans and some
hydras have limited
locomotion.
• This is provided by the
antagonistic circular
and longitudinal fibrils
(derived from
myoepithelial cells)
These work in conjunction with the gastrovascular cavity
as a hydrostatic skeleton

15. 4. Alternation of Generations
The Polyp and Medusa body forms
Medusa form:
• Bell- shaped or umbrella
shaped body, with the mouth
directed downward
• Small tentacles directed
downward
• Possess a large amount of
mesoglea
• Motile, move by weak
contractions of body
• Polyp form:
• Tubular form with the mouth directed upward
• Around a mouth are a whorl of feeding tentacles
• Only have a small amount of mesoglea
• It is sessile

16. Generalized Cnidarian Life Cycle
• The life cycle of cnidarians often includes an alteration
of sexual and asexual generations, - a phenomenon
that is referred to as metagenesis.
• However fresh water hydra and some marine
hydroids do not have a medusa stage.
• When metagenesis occurs, the polyp is the asexual
generation and the medusa is the sexual generation.
• A generalized life cycle occurs as follows:
• Medusae produce gametes which unite to form
zygotes.
• Each zygote divides repeatedly and develops into a
free-swimming larval form called a planula larva
• The larva then settles and grows into a polyp (asexual
generation).
•

17. Cnidaria reproduction cont..
• Each polyp then asexually produces medusae by
budding to complete the life cycle.
• This generalized life cycle is modified greatly in
different groups
• Either the polyp or the medusa stage may be
reduced or even completely absent from the life
cycle.
• When the medusa stage is absent from the life cycle,
the polyp reproduces both asexually and sexually.

18. Generalized Cnidarian Life Cycle

19. Strobilation in cnidarians
• “Strobilation” = asexual
reproduction consisting of
the spontaneous
transverse segmentation
of the body.
• The 2 processes occur
simultaneously with
segmentation to release
new ephyras.

20. Cnidaria Classification
• Four classes are recognized:
• (a) The Hydrozoa to which Hydra and Obelia belong.
• (b) Scyphozoa, the jellyfish, including Aurelia
• (c) Anthozoa, the sea anemones and corals.
• (d) Cubozoa, the box jellyfish & sea wasps)

21. Class Hydrozoa
• Examples: Obelia, Portuguese Man-of-War, Hydra
• Some species live as polyps, some as medusae, some alternate
between forms.
Hydra Portuguese man of war
Fire coral

22. Class Hydrozoa
• Includes the solitary fresh water hydra (Hydra),
Obelia, fire coral (Millipora) and the Portuguese
Man o' War (Physalia),
• Most are colonial and marine, with some fresh
water forms.
• Typical life cycle includes both asexual polyps
and sexual medusa stages
• However fresh water hydra and some marine
hydroids do not have a medusa stage.
• The polyp is sessile and usually colonial in that
sexual budding can take place but the
individuals so formed remain attached to each
other.

23. Class Hydrozoa cont……
• The medusa is a free swimming member of the
plankton and represents the sexually
reproducing, dispersal phase, in a typical
hydrozoan.
• Medusa when present, with a velum, gonads
forms in the ectoderm
• Hydra is an exceptional member of this class in
that it only occurs as a polyp, which can
reproduce sexually.
• Most colonies exhibit polymorphism in the polyp
types

25. Obelia polyps
• All polyps in the colony
are interconnected
• Two different kind of
individuals that comprise
the colony
Feeding polyps or
gastrozooids (C)
Reproductive polyps or
gonozooids (B)

26. Longitudinal section of Hydra

27. Typical structure of a Hydroid polyp

28. Hydra
• Found in quiet ponds, lakes, streams.
• Attach to rocks or water plants by means of
sticky secretion by cells at base.
• Can leave one location and move to another by
secreting bubbles at base causing it to float to
surface or can move by somersaulting motion
• Reproduce asexually during warm weather by
budding on side of body.

29. Life cycle of Hydroid (Obelia)
• Starts with sessile,
asexually reproducing
polyp colony
• Polyps release free
swimming medusae
(asexually, by budding).
• The medusae reproduce
sexually, releasing sperm
and eggs that fertilize to
form a zygote.
• Zygotes morphs into a
blastula, then a ciliated
planula larvae which
eventually settle to
become polyp colonies.

30. Portuguese man-of-war, Physalia
otricus
• Is a hydrozoan cnidarian that has adopted the
way of life characteristic of the jellyfish.
• Are the most complex and specialized
hydrozoans consisting of colonies of highly
specialized individuals.
• Pelagic colonial marine animals (polymorphic).
• Body consists of both modified medusoid and
polypoid individuals ‘zooids’ that are
morphologically and functionally specialized.
• Polyps specialized for feeding, digestion, or
sexual reproduction
• Gastrozooids take in and digest food and from
here it is distributed through the whole colony

31. Class Anthozoa
 Are sessile polyps with expanded and flattened
oral disc.
 Includes sedentary/solitary polyploid forms such
as sea anemones, sea fans and colonials e.g.
corals.
 Only polyp forms, no medusa stage.
 At one or both ends of the mouth is a ciliated
groove called the siphonoglph; generates a
water current and brings food to the
gastrovascular cavity.

32. Class Anthozoa
Are the most advanced members of the
phylum Cnidaria
Occur in both deep and shallow marine
waters.
Corals obtain much of their energy from
microscopic photosynthetic green algae
(zooxanthellae) or dinoflagellates that live
symbiotically inside the cells of the corals.

33. Class Anthozoa
- anemones and true corals
-exist only as polyps - medusa stage is
absent
Capnella Soft Coral
Anthozoa Colony

34. Cnidarian - zooxanthellae symbiosis
• Because the
zooxanthellae are
plants, they need
sunlight to
photosynthesize
• Some cnidarians are
nearly completely
dependent on
zooxanthellae
• Not all corals are
dependent on
symbionts
• These live at great
depths where there is
low or no light
zooxanthellae in the soft coral Xenia

35. Coral Reefs
• Colonial, reef-forming corals
depend on zooxanthalae;
thus, reefs can only exist in
shallow water.
• Corals depend on algae
to provide oxygen and
to speed up
accumulation of calcium
from the sea water.
• Algae depend on corals
to supply vital nutrients.

36. Class Scyphozoa
• Commonly known as jellyfish e.g Aurelia
• 200 species.
• “Cup animals”. Medusa is the dominant
form of the life cycle - polyp stage is
reduced.

38. Class Scyphozoa
• In these animals the medusa is the dominant
while polyp form is reduced (restricted to a small
larva) or absent.
• Polyp called a scyphistoma, undergoes
strobilation to produce ephyra which are
immature medusa stages.
• Sexual reproduction involves gamete production
by free-swimming medusa.
• The medusae are large and contain massive
amounts of mesogle
• Solitary
• Gonads form in the endoderm of medusa.

39. Life cycle of a scyphozoan (jellyfish)
• They eventually undergo multiple transverse
fissions through a process called “Strobilation”
>a process by which new medusae are
produced'
• The resulting Strobila sheds immature jellyfish
(Ephyra) at the oral (tentacular) end. Ephyra
eventually mature into adult jellyfish
• Depending on the species, a single polyp may
produce one or many ephyrae all at once, over a
period of time, or at different intervals.
• The ephyra subsequently develops into a
mature medusa over a period of weeks to
months

40. Generalized life cycle of a
scyphozoan jellyfish, Aurelia.

41. Scyphozoan feeding strategies
• Some species of jellyfish do not use their nematocysts
for food capture
• Instead feed on suspended zooplankton and
organic matter, drawn into the coelenteron by
currents of water generated by flagellae of the
gastrodermal cells.
• Others use nematocysts, which can be charged with
venom.
• The most potent is found in the tropical sea wasps or
box jellies (Cubozoans).
• A sting from a sea wasp can cause death in humans
within twenty minutes!
• Scyphozoans and siphonophores are predators of fish
and small invertebrates

42. Class Cubozoa
• Box jellyfish and sea wasps belong to this class.
• Both polyp and medusa forms are present (medusa =
dominant, polyp = reduced)
• Polyp transforms directly into medusa stage without
strobilization
• Solitary
• Medusa without a velum, but with a velarium
• Medusa = sexual generation, gonads forms in the
endoderm
• All are marine

43. Summary on Cnidaria
• Members of Phylum Cnidaria are radially or
biradially symmetrical
• They also possess diploblastic tissue level of
organization
• Cells are specialized for food gathering,
defence, contraction, coordination, digestion
and absorption
• Hydrozoans differ from other cnidarians in
having extodermal gametes
• Also have mesoglea without mesenchyme cells
and nematocysts only in their epidermis

44. Summary on Cnidaria
• Most hydrozoans have well developed polyp
and medusa stages
• Class Scyphozoa contains the jellyfish
• The polyp stage of scyphozoans is usually very
small.
• Members of Class Cubozoa live in warm,
tropical waters, some posses nasty
nematocysts
• The Anthozoa lack the medusa stage. They
include sea anemones and corals

45. Part 2: PHYLUM CTENOPHORA
General Introduction
• Ctenophorans are commonly called Comb jellies
• About 90 species with worldwide and marine
distribution.
• Ctenophora means “comb-bearer” (Greek word) and
these animals are named for their eight rows of comb-
like plates composed of fused cilia arranged along the
sides of the animal
• There are nematocysts in one group of ctenophores
(Euchlora rubra) (order Cydippida)
• This shows that ctenophores may have arisen from a
Medusa ancestor.
• However, the relationship between the 2 phyla is
uncertain.

46. PHYLUM CTENOPHORA
• The interesting behavior in comb jellies is their
bioluminescence (cold light)
• The light comes from the gastrodermis cells in the
walls of the meridional canals.

47. –Bioluminescence: production of light. Often
observed in large swarms near surface at night.

48. 2. Distinguishing Characteristics of
Phylum Ctenophora
• Ctenophorans are diploblastic i.e have two embryonic
germ layers
• The outer layer (epidermis) and inner layer
(gastrodermis) by jell-like material called mesoglea
• Have biradial(radial + bilateral) symmetry with an oral-
aboral body axis.
• They lack nematocysts (except Euchlora rubra) but have
2 tentacles with specialized adhesive exocytotic
structures called colloblasts
• These are “glue cells” used to capture prey (feeding).
• Have only one “body cavity” gastrovascular cavity in
form of canals.

49. Distinguishing Characteristics of Phylum
Ctenophora…cont….
• No specialized gas exchange, circulatory or
excretory systems.
• Sub epidermal nerve network (nerve net) is best
developed under the comb rows and more
specialized than in Cnidarians
• Body support is through elastic mesenchyne
(gelatinous, makes up most of the body mass;
mostly water) with muscle cells
• Planktonic, neutral buoyancy maintained
through passive osmotic adjustment.

50. 3. Classification of Ctenophora
• At least 100 species have been described.
• They are classified on the basis of their
tentacles, body form, oral lobes and body
compressions.
• There are 2 classes and 6 orders in the phylum.
• Class Tentaculata (with tentacles)

51. Class Tentaculata (with tentacles)
• All in this class have two tentacles; a characteristic
species is Hormimorpha plumosa.
• Some in this class are flattened into a ribbon-like
shape along the plane of the tentacles and have an
undulatory motion. i.e. Cestium sp.

52. Class Nuda (without tentacles)
• These do not have tentacles at any
stage in their lives
• The characteristic species is Beroe
punctatus.
• Have a wide, flexible mouth on their bell
shaped bodies.
• Look hollow & can eat prey larger than
themselves.
• Have hooks on their gullet to allow them
to hold their prey
• Also have glands that secrete a poison
that paralyzes their prey
• Half of combjelly spp belong to Class
Nuda

53. 4. Ctenophora Feeding and
Nutrition
• All ctenophores are predatory (carnivorous),
feeding on zooplankton including copepods,
small crustaceans, larval fish and fish eggs .
• Comb jellies either actively seek their prey or
wait in ambush for it.
• Prey is captured by long tentacles equipped
with “sticky cells” colloblasts
• When the tentacles touch prey, the sticky cells
burst and discharge a strong, sticky material.
• Then the tentacle is pulled up to the mouth and
wiped off.
•

54. Feeding and Nutrition…………..
• The food starts to be digested extracellularly in the
pharynx.
• Partially digested food is then distributed throughout
the body by complex systems of gastrovascular
canals.
• These canals are lined with endodermal cells that
complete digestion of food materials intracellularly.
• The nutrients are absorbed via pinocytosis and
phagocytosis

55. 5. Reproduction in Ctenophora
• Ctenophora reproduce sexually, with the
exception of species of the order Platyctenida.
• These reproduce asexually by regeneration of
an entire organism from a small piece of the
adult body.
• Most species are hermaphroditic, but some
gonochoristic.
• In most, gametes are shed through mouth.
• Most comb jellies shed their eggs and sperm
into the sea water.
• Fertilization and development are external.

56. Reproduction……cont….
• In some species, fertilization takes place
inside the body (internal fertilization)
• Self-fertilization is somewhat rare and is
known only to appear in the genus of
Mnemiopsis.
• The larvae (cydipid) swim freely during their
transformation into adults.

57. Ctenophore life cycles
• Ctenophores carry out their entire life cycle in the
plankton, so they are considered to be
"holoplanktonic".
• Most, but not all, species of ctenophores are
hermaphroditic, meaning that a single ctenophore
carries both male and female gonads.
• Ctenophores spawn these eggs and sperm freely
into the sea, where the sperm must first find the
eggs for fertilization
• Fertilized eggs develop into a larvae called a
Cydippid larvae.
• This free swimming larvae grows naturally into a
new Comb jelly.

58. Locomotion in Ctenophora
• Neutrally buoyant, but swim using comb rows with
“ctenes” (transverse bands of partially or wholly
fused cilia beating synchronously.
• May have two long tentacles or no tentacles

59. Circulation, Gaseous exchange and
sensitivity in ctenophorans
• No circulatory system
• Gaseous exchange is by simple diffusion. Occurs at
gastrovascular or epithelial surface.
• Primitive Nervous system – “no brain”, just a
network of neurons (Nerve net).
• Have sense organ (statocyts) situated apically -
near the anus containing a calcareous statolith.
• These specialized cells that help them maintain
vertical position & orientation when swimming e.g.
jellyfish.
• Some have chemical sense organs around their
mouth, e.g class Nuda use chemicals to detect
presence of other Ctenophores.

60. 7. Summary on Ctenophora
• Members of Phylum Ctenophora are
biradially symmetrical and diploblastic
• Bands of cilia called comb rows
characterize ctenophore
• The Cnidaria and Ctenophora are distantly
related phyla.
• Within the Cnidaria, the ancient hydrozoans are
believed to be the rock from which modern
hydrozoans and other cnidarians evolved.

61. 8. Differences between Ctenophora
and Cnidaria
Ctenophora
 Cydippid larva
 No nematocysts except in
Euchlora rubra, instead have
colloblast cells
 Typically hermaphrodite
 Multiciliated cells
 No alternation of generations
 Have apical organs:
 Bi-radial symetrical
Cnidarians
 Planula larva
 Have nematocyts
 Typically gonochoristic
 Monociliated cells
 Alternation of generations
occurs
 No apical organs
 Radial symetrical