1. PHYLUM CNIDARIA
ZLY 303: Biology of Free-living Non-Arthropods

2. Subkingdom: Eumetazoa Phylum: Cnidaria
Classification
Domain: Eucaryota
Kingdom: Animalia
Subkingdom: Eumetazoa
Phylum: Cnidaria
Cnidarians were classified into the following
Subphyla viz:
Anthozoa (Sessile) e.g. Sea anemones,
Corals, Sea pens
Ceriantharia (tube anemones),
Hexacorallia,
Octocorallia,

3. Staurozoa (Stalked Jellies) Myxozoa (Parasites)
Classification
Medusozoa
Scyphozoa (Swimming) e.g. true jellyfish
Cubozoa e.g. box jellyfish, Sea wasps
Hydozoa (Sessile) e.g. Hydra and colonial swimmers e.g. Portuguese Man o’ War (A diverse group that include all freshwater cnidarians as well as many marine forms).
Recently, the following group has been recognized as a class rather than a sub-group of Scyphozoa; they include:
Staurozoa (Stalked Jellies)
Myxozoa (Parasites)
Polypodiozoa (Parasites)
There is still debate whether they are Cnidarians or Bilaterians

4. Phylogeny of Cnidaria
Four main classes (even though most sources consider 3) stem group - possess basic structural organization of other more advanced metazoa
have radiated into many habitats – yet body plan has been retained.

5. Classes
For study purpose, the following classes shall be considered:-
HYDROZOA e.g. Hydra, Obelia
SCYPHOZOA e.g. Aurelia aurita (Jelly fish)
CLASS ANTHOZOA e.g. Sea anemones, Corals
CLASS CUBOZOA e.g. Tripedalia cystophora, Sea wasp

6. General Characteristics
Possess the simplest degree of structural and functional organization
Exclusively marine, few sp. in freshwater
They are acoelomates (lack coelom)
Radially symmetrical
There is diversity of body types found among the members of this phylum.
Has 2 distinct layers i.e. Ectoderm and Endoderm and a thin layer of non-living jelly-like mesoglea

7. General Characteristics
Tentacles around the mouth
Tissues level of organization
No choanocytes (flagellated collar cells of the sponges)
Alternation of generation with 2 morphologically distinct stages:
Polyp (sessile, remain attached, asexually reproducing form)
Medusa(motile, sexually reproducing form)

8. General Characteristics
No excretory or respiratory organs
Nerve net with symmetrical & asymmetrical synapses
Gastrovascular cavity used for both digestion and excretion.
Mesoglea (jelly-like non-living substance between the two tissue layers (epidermis & gastrodermis) for support).
Hydrostatic Skeleton - chitinous, calcerous, or protein.

9. General Characteristics
Radiata have only two embryonic tissue layers.
Specifically, no mesoderm (the tissue that gives rise to structures, including muscles, in triploblastic organisms).
Lack sophisticated movement seen in triploblastic organisms.
Mouth: both ingestion & egestion- no anus
Asexual reproduction by budding in polyps
Sexually by gametes in all medusa & some polyps

10. Body Cavity Diversity

11. Why Cnidarians are ACOELOMATES
Cnidarians do not possess the following features, hence, they are Acoelomates:-
Coelom (also called the body cavity) cavity within the body of all animals higher than the coelenterates and certain primitive worms.
Formed by the splitting of the embryonic mesoderm into two layers. 
In mammals it forms the peritoneal, pleural, and pericardial cavities. 

12. Dimorphism & Polymorphism in Cnidarians
A striking & puzzling feature
Dimorphism: existence in polyp & medusa
Polymorphism: colonies of some species have morphologically differing individuals each specialized for certain roles e.g. feeding, reproduction & defense.

13. Task differences in Hydrozoa
In Hydrozoans, colonial individuals arising from individual zooids will take on separate tasks. For instance, in Obelia there are:
Feeding individuals referred to as the Gastrozooids;
Asexual reproduction individuals only, referred to as Gonozooids;
Sexually reproducing individuals (medusae) and are free-living, referred to as the Blastostyles
Defensive individuals referred to as Dactylozooids

14. Polymorphism in Cnidaria

15. Differences between Medusa & Polyp
Sedentary or sessile life
Non free swimming
Has tubular body
Simple body structure
Velum absent
Mouth is circular
Mesoglea poorly developed
Adapted for a floating life
Free swimming
Bell or umbrella-shaped
Complex structure
Velum present
Mouth rectangular
Mesoglea highly developed

16. Attack & Defence Feature
Cnidarians also have cnidocytes (specialized cells that function in defense & capture of prey)
Cnidocytes contain organelles called cnida, which are able to evert (extend or retract).
Cnida that sting are called nematocysts.
These nematocysts can immobilize fish for capture, and they can also be used for defence.

17. Coiled & Uncoiled Nematocysts

18. Types of Cnidocytes
Nematocysts: They have barbs which are used to inject venom into prey. This is common to almost all Cnidarians.
Spirocysts: They entangle the prey and do not penetrate and inject the prey with venom.
Ptychocysts: They have tiny threads used for building protective tubes for the organism to habitate; they are not used for prey capture, this feature is common to tube anemones.

19. Features of a Cnidocyte
Cilium: A fine hair which projects above the surface and acts as a trigger. They are absent in Spirocysts.
Cnida: A tough capsule which houses the thread (payload) and contains mixture of chemicals like venom, adhesives etc.
Finger-like-extensible tube: Found on the wall of the cnida so that when a cnidocyte fires, the finger pops out and a set of barbs anchor the prey to release venom.

20. Features of a Cnidocyte
The thread: An extension of the "finger" and coils round the prey until the cnidocyte fires. (The thread is usually hollow and delivers chemicals from the cnida to the target).
Operculum: A lid covering the end of the cnida. (The lid may be a single hinged flap or three flaps arranged like slices of pie).
Cell body: Produces all the other parts.

21. Firing Mechanism of Cnidocytes
Four theories were postulated by Hinde (1998):
Rapid contraction of fibers round the cnida may increase its internal pressure.
The thread may be like a coiled spring that extends rapidly when released.
Chemical changes in the cnida's contents may cause them to expand rapidly by polymerization e.g. Chironex (the "sea wasp").
Chemical changes in liquid of the cnida increases the concentration, so that osmotic pressure forces water in very rapidly to dilute it e.g. Hydrozoa,

22. Figure Showing Firing Sequence
Operculum (lid) opened
Barb
“Finger” turns out
Victim's skin
Filament
Victim's tissues

23. Stimulus Required to Avoid Wastage in Firing Cnidocytes
Their cilia detect contact, and nearby sensory cells "smell" chemicals in the water. This combination prevents them from firing at distant or non-living objects.
Groups of cnidocytes are usually connected by nerves and, if one fires, the rest of the group requires a weaker minimum stimulus than the cells that fire first.

24. The Hydrozoa e.g. Hydra vulgaris
Class Hydrozoa includes hydras and the infamous Portuguese Man-of-War, an organism noted for its potent sting, in the medusa form.
Most Hydrozoans exist in the polyp and medusa forms.
For example, some hydrozoans e.g. Obelia exist as an asexually reproducing polyp that alternates with a sexually reproducing medusa form (see next slide).

25. Body plan of Hydra

26. Body plan of Hydra Epidermis consist:
Epitheliomuscular cells (can self-reproduce) for covering & muscular contraction
Interstitial cells: differentiate to form the cnidoblast, sex cells, buds, nerve cells.
Gland cells- located around the basal disc and mouth, secrete adhesive substance for attachment
Cnidocytes- contain nematocyst for catching prey
Sensory cells found around the mouth for detection of stiimuli
Nerve cells- synapse with sensory cells & other nerve cells

27. Body plan of Hydrozoa: Hydra

28. Transverse Section through body wall of Hydra

29. Body plan of Hydra Gastrodermis consist:
Nutritive-muscular cells all tall, columnar, large and vacuolated with flagella at one end & microvilli at the other end
Interstitial cells: can transform to form other kind of cells when the need arises.
Gland cells- surround the mouth to aid feeding & digestion.
Mesoglea lies btw the epidermis & dermis and act like a type of elastic skeleton

30. Biology of Hydra: Hydrozoans
May be solitary or colonial
Found in freshwater
Hydra, unlike other hydrozoa exist as solitary(no medusa or colonial forms).
Cylindrical body shape
Body size depends on level of contraction and extension.

31. Movement in Hydra
Somersaulting- Can be found hanging downward
Walking
Looping
Floating
Climbing
Gliding
Swimming

32. Feeding & Nutrition In Hydra
Use Tentacles to capture & direct the food
Exclusively carnivorous feeds on Cyclops, annelids, crustaceans, insect larvae.
Digestion in enteron (gastrovascular cavity)& is both
Extracellularly- the prey is killed by the action of digestive enzymes from gland cells of gastrodermis
Intracellular- occur in the gastrovascular cavity

33. Feeding & Nutrition In Hydra
Tentacles have many cnidoblasts in them for capturing prey
Nematocysts: Immobilize & paralyze the prey
Inside the gastrovascular cavity, gland cells secrete enzyme to digest the food.
Egestion via the mouth.

34. Feeding process in Hydra

35. Reproduction in Hydrozoa: Hydra
Mostly dioecious(male & female parts separate
Reproduce both sexually & asexually by budding
Zygote undergo holoblastic(complete) cleavage to form the blastula.
Cleavage is indeterminate(regulative)
Fertilization: Cyst formed around the embryo which break loose from parent to form the young hydra.

36. Reproduction in Hydrozoa: Hydra

37. Biology of Obelia
Unlike Hydra, has both colonial & medusa form
Sedentary
It’s a plant-like Hydrozoa
Whitish in colour under microscope
Largely carnivorous
Use tentacles to capture prey
Digestion both extra & intracellular

38. Biology of Obelia
Dioecious
Fertilization is external
Mouth open into manubrium
Zygote forms planular larva
Statocysts are gravity receptor for maintenance of balance

39. Obelia life cycle

40. The life cycle of Obelia, (Hydrozoan)

41. Alternation of Generation in Hydromedusoid forms

42. Cnidaria Larva Types Planula of Renilla sp.
Planula of Anemone Lebruina
Brooded Planula of Urticinia filina (anemone)
Planula of Zoanthidea
Planktotropic planula M. senile
Early & Late planula of stone coral
Actinula larva Ceriantheria

43. Body plan of Scyphozoa: Aurelia aurita

44. Life Cycle of Jelly fish

45. Class Scyphozoa e.g. Jellyfish
Cup-animals with umbrella or bowl-shape
May attain a bell diameter of more than 2m
Most scyphozoans range from 2 to 40cm in diameter
May be colourless, orange or blue
Jellyfish exists predominantly in the medusa form
Mesoglea unusually thick giving the bell a fairly firmed consistency

46. Biology of Class Scyphozoa e.g. Jellyfish
Predominantly Medusa, polyp form occurs only as small larva during life cycle stage.
Thaumatosyphus hexaradiatus are unusual member because medusa are sessile, attached to seaweed
Movement is by rhythmical pulsation of the umbrella
Tentacles may be few or short as in Aurelia aurita or long as in Cyanea

47. Biology of Class Scyphozoa e.g. Jellyfish
Aurelia aurita is a common member
No velum or nerve rings
Oral arms bears the nematocysts
Have sense organ called rhopalium
Nervous system is made of a nerve net with subumbrella net that control bell pulsations

48. Biology of Class Scyphozoa e.g. Jellyfish
Sexes separate
Sperm carried by ciliary current into gastric pouch of female
Internal fertilization
Reproduction: The medusa sperm + egg = Zygote
Ciliated planular larva emerge (Scyphystoma) (hydra-like form), strobila ephyra

49. Class: Anthozoa Sea anemone

50. Class Anthozoa
E.g. sea anemones, corals , and sea fans.
Anthozoans are all polyps, no medusa i.e. Not Dimophic
Symbioses between reef-building corals and the dinoflagellates.
In these relationships, corals provide housing and protection for the protists, and the dinoflagellates provide food for the corals.
Members of all of the Cnidaria classes can respond to external stimuli and can use stinging nematocysts for prey capture and defense. 

51. Anthozoa characteristics
Flower-shaped, with body far more complex than Hydrozoans
Exclusively polyploidy, no medusa
The most advanced of all marine Cnidarian
Mouth lead to pharynx
Gastrovascular cavity subdivided by septa, or mesenteries into chamber
Mesoglea stout with cells and fibres
Skeleton secreted in the form of coral
Nematoblast found in the body layers

52. Class Anthozoa e.g. The Corals

53. Biology of Anthozoans
Feeding
Similar to Hydra polyp
Aided with tentacles, cnidoblast and nematocyst.
Pharynx lined with lateral ciliated grooves
Movement by rhythmic contraction of muscles, mesoglea and water in gastric cavity

54. Biology of Anthozoans e.g. Sea anemones
Move by gliding slowly along the substrate on their pedal disc
Are carnivorous
Can expand & stretch their tentacles to catch the prey
Nematocysts immobilizes the prey
Pharynx with ciliated grooves: siphonoglyps
When disturbed, can contract & withdraw their tentacles and oral discs.

55. Biology of Anthozoans e.g. Sea anemones
Sexual Reproduction
Sexes are separate
Gonads borne on the margin of septa/mesenteries
Fertilization occurs in enteron or externally:
Planular larva emerge
Asexual Reproduction
Occur by pedal laceration or at times budding

56. Diversity in forms HYDROZOA
1.Life history features both Polyp and Medusa stages
2. Mesoglea non cellular
SCYPHOZOA
True Jellyfishes have conspicuous medusoid form, Polyploid larva stage
2. Mesoglea thickened with fibres and cells
ANTHOZOA
Sea anemones, coral are all polyps
2. Mesoglea stout with fibres and cells
CUBOZOA
Medusa predominant

57. Class Cubozoa
Class Cubozoa are the box jellies, which have a box-shaped medusa form.
Some species in this group are among the most venomous organisms in the world and their stings can be fatal to humans

58. Class Cubozoa
Most recently classified
Exist as both Polyp & Medusa
Medusa is the predominant form of this class
Cubozoa medusa may be about 25cm tall
Medusa are strong swimmer and voracious predators
Base of each tentacle form pedalium
Tentacle(s) present in each corner of umbrella margin

59. Class Cubozoa Medusa dominant Polyp state develop directly to medusa
Bell margin with velarium
Tetramerous (four part radial symmetry)
Bell may be cube or square-shaped with tentacles arising from each corner.
Gonads are endodermal (found in the gatrodermis)
Specialized sense organ called rhopalium

60. Class Cubozoa