3. CNIDARIA Phylum

Jellyfish and cousins (Hydras, Medusae, Anemones, Corals)

Cnidaria Phylum (Hydras, Medusae, Anemones, Corals)

• Diploblastic

  • Ectoderm —> Epiderm

  • Endoderm —> Gastroderm

• Characteristic cell types for stinging

  • Cnidocyte (cell type) with Nematocytes (stinging organelles)

• DIFFERENTIATED Cell Types

  • Nerve cells (nerve net)

  • Contractile epithelial cells

  • Enzyme-secreting cells

  • Nutrient absorbing cells w/ intercellular connections

• LACK of organ formation

  • Exceptions: Statocysts and rhopalia

• Radial or biradial symmetry

Body Plans: Polyp vs. Medusa

Cnidaria exist in two primary forms during their lifetime

1. Polyp (Sessile)

2. Medusa (Motile)

Three Layers and One Cavity:

1. Epidermis

2. Gastrodermis

3. Mesoglea

4. Gastrovascular Cavity

Polyp (Sessile)

• Mouth/anus facing UPWARD

• Attached to substrate

• Cylindrical body

Medusa (Motile)

• Mouth/anus facing DOWNWARD

• Free-swimming “jellyfish” form

• Umbrella-shaped (bell)

3 Layers & 1 Cavity of Polyp and Medusa

Three Layers and One Cavity:

1. Epidermis

2. Gastrodermis

3. Mesoglea

4. Gastrovascular Cavity

Epidermis

OUTER layer derived from ectoderm

Gastrodermis

INNER lining of gut derived from endoderm

Mesoglea

“Jelly” middle

Non-cellular gelatinous layer between the two tissues layers

Gastrovascular Cavity

Central digestive compartment

SINGLE opening

• Functions as BOTH MOUTH AND ANUS

  • “Incomplete” digestive system

Life Cycle of Obelia (Hydrozoa)

Obelia is dimorphic, meaning it uses both body plans to complete its life cycle.

1. The Colony: A branched network of polyps. They are interconnected and share a gastrovascular cavity.

2. Polyp Specialization:

   • Gastrozooid: Feeding polyp with tentacles.

   • Gonozooid: Reproductive polyp. It lacks tentacles and produces medusae through asexual budding.

3. Medusa Phase: The tiny medusae swim away and represent the sexual phase. They produce gametes (eggs and sperm) via meiosis.

4. Fertilization: Sperm and egg meet in the water to form a zygote (2n).

5. Planula Larva: The zygote develops into a solid, ciliated larva that swims until it finds a place to settle.

6. New Colony: The larva settles on the ocean floor and grows into a new polyp colony.

Cnidocytes; Stinging

*Cnidarians are named after their CNIDOCYTE

• Specialized cell for DEFENSE and CAPTURING PREY

Cnidocyte

Entire cell used for defense and capturing prey

• CANNOT BE REUSED

  • Animal must grow a NEW CNIDOCYTE

Nematocyst

Capsule INSIDE the cell that HOLDS a COILED, BARBED THREAD

Cnidocil “Trigger”

Hair-like projection

When touched (chemically or physically), it causes the nematocyst [capsule] to fire

Discharge

Thread turns inside out and shoots out

Barbs puncture prey and thread delivers toxins to paralyze

Classes of Cnidaria Phylum (2)

1. Medusozoa (Medusa, motile)

2. Anthozoa (Polyp only, sessile)

Medusozoa (2 Monophyletic Clades)

Polyps and medusoids, motile

2 Monophyletic Clades:

1. Hydrozoa

   • Sessile forms

   • Medusoid forms

   • Swimming colonial forms

     • Portugese man o’ war

2. Scyphozoa and Cubozoa

Anthozoa

Only sessile polyp forms

• Corals, sea fans, sea whips, sea anemones

Class Anthozoa (Sea Anemones)

Only sessile polyp forms

• Cylindrical

• Bilateral

• Feed on small fish and shrimp

• Mutualistic relationship with hermit crab

Class Anthozoa (Corals)

Only sessile polyp forms

• Some form exoskeleton with CaCO3

• Smaller colonial polyps

• Feed on plankton

Class Scyphozoa (Medusozoa)

Polyp and medusoid forms

Marine jellies, “cup animals,” bell shape

• Small

• MEDUSA FORM IS PROMINENT

• Free-swimmers

• Solitary carnivores

• Present nerve cells

  • Nerve net around bell

  • Some show rhopalia (cluster of sensory organs)

• Ring of muscles lines dome of body (contractile force)

Life Cycle of Scyphozoa (Medusozoa)

Polyp and medusoid forms

Marine jellies, “cup animals,” bell shape

Dioecious

• Both male and female reproductive organs

• Gonads form from gastrodermis and are expelled through mouth

Life Cycle:

1. Egg + Sperm

2. Zygote

3. Planula larva

4. Polyp

5. Budding Polyp

6. Ephyra

7. Medusa

Class Cubozoa (Medusozoa)

Polyp and medusoid forms

“Box jellyfish”

• Morphologically similar to Scyphozoa BUT TENTACLE ARRANGEMENT IS DIFFERENT

Pedalia (PE)

• Muscular pads at corners of bell canopy

Rhopalia (RO)

• Cluster of sensory organs

• Nemocysts spirally arranged among tentacles

• Dioecious

  • Planula larvae may develop within female

• FORM REAL ORGANS

  • Complex eyes (navigation/hunting)

Class Hydrozoa (Medusozoa)

Polyp and medusoid forms

Portuguese man o’ war

• SIPHONOPHORES

  • Free-floating COLONIAL (both forms)

    • See picture

• Marine (most), some freshwater

• Polypoid and medusoid forms

  • Hydra sp. only polypoid

Class Hydrozoa (Medusoid)

Polyp and medusoid forms

Hydra

Anatomy

• Epidermis (Outer)

  • Nerve cells, part of nerve net

  • Stinger capsules (cnidocytes)

    • Don’t have a brain

• Gastrodermis (Inner)

  • Digestive cells

    • With flagella circulates food

  • Food vacuoles

    • Intracellular digestion

• Nerve Net

  • Red nerve cells on eppidermis

  • React to stimuli from ANY direction

• Digestion Flow

  1. Gastrovascular Cavity (Extracellular digestion)

     • Enzymes break food down

  2. Digestive Cells (Intracellular digestion)

     • Occurs in gastrodermis