1. Lecture 6: Phylum Cnidaria, Part 1
Invertebrate Zoology
Lecture 6: Phylum Cnidaria, Part 1

2. Lecture outline Overview Overall body structure
Body wall
Cnidocytes
Bauplan, polyp (Anthozoa)
Bauplans, medusae
Hydrozoa
Scyphozoa
Cubozoa
Focus on nervous systems/sensory structures

3. Overview Thought to be plants until 18th century
Two basic body forms  diverse array of organisms

4. Overview Four classes Class Hydrozoa [Clockwise from top] Hydroids
Hydromedusae
by-the-wind sailors
Siphonophores
Siphonophore Photo:
K. Raskoff / Monterey Peninsula College

5. Overview
Four classes
Class Scyphozoa
Large jellies
Stauromedusae

6. Overview Four classes Class Cubozoa Box jellies

7. Overview Four classes Class Anthozoa Anemones Coral Sea pansies
Gorgonians

8. Overview Evolutionary relationships (briefly)
Thought to have arisen relatively early in animal evolution—one of the longest fossil histories
Key derived characters
diploblasty
radial symmetry
Was the first cnidarian a polyp or a medusa?

9. Overview Key characteristics (see Box 8A)
Diploblastic; epidermis & endodermis
Mesoglia/mesenchyme between tissue layers
Radial symmetry (may be biradial, etc…)
Possess cnidocytes
“Muscles” associated with epidermis, endodermis
Many alternate between asexual polyps and sexual medusae
Contain gastrovascular cavity, not a true body cavity
No head; no special body systems for circulation, gas exchange or excretion.
Nervous system is a nerve net
Have planula larvae

10. Overall body structure
Body wall
Epidermis
Epitheliomuscular cells:
Contractile portion at base (=myoneme)
Epidermal gland cells
Sensory cells
Nerve cells
Cnidocytes

11. Overall body structure
Body wall
Gastrodermis
Nutritive-muscular cells
Have myonemes
Enzymatic gland cells
Mucus gland cells
Nerve cells
Cnidocytes
Mesoglia/mesenchyme
Between epidermis and gastrodermis

12. Overall body structure
Cnidocytes
Functions
Cnidae
Focus: nematocyst-bearing cnidocyte
Cell body
Cnidocil: how triggered?
Operculum

13. Overall body structure
Cnidocytes
Focus: nematocyst-bearing cnidocyte
Nematocyst capsule
With toxins
Shaft
Stylets and spines
Function?
Hollow filament

14. Overall body structure
Cnidocyte: firing
Trigger cnidocil
Eversion of shaft and filament
Injection of toxins
Hypotheses
Osmotic hypothesis
Tension hypothesis
Contractile hypothesis

15. Overall body structure
Cnidocytes
Other cnidae (adhesive)
Spirocysts
Ptychocysts (no image)

16. Bauplan (polyp, Anthozoa)

17. Bauplan (polyp, Anthozoa)
Tentacles
Not true appendages (why not)
Contain branches of the g.v. cavity
Epidermal layer with cnidocytes
Acrorhagi: specialized defensive tentacles

18. Bauplan (polyp, Anthozoa)
Oral disc/ mouth/ pharynx
Epidermis extends down pharynx
NOTE: Hydrozoan polyps lack a pharynx.

19. Bauplan (polyp, Anthozoa)
Siphonoglyph
Ciliated grooves: create water current
Often have two
Water flow pattern?
Reversal of flow?

20. Bauplan (polyp, Anthozoa)
Gastrovascular cavity
Functions
Movement into g.v. cavity
Movement within g.v. cavity
What causes fluids to circulate?
Mesenteries
Folds of gastrodermis
Divide the g.v. cavity
Function?
Some complete, others not

21. Bauplan (polyp, Anthozoa)
Gastrovascular cavity
Mesenteries (x-sec)
Cnidocytes associated with mesenteric filaments.
Acontia: Free-hanging filaments; defensive
Gonads associated with mesenteries (Anthozoa)
Myonemes

22. Bauplan (polyp, Anthozoa)
Column/Pedal disc
Attachment
Epidermal gland cells
Movement
Why move?
Sessile polyps

23. Bauplan (polyp, Anthozoa)
Column/Pedal disc
Movement (cont.)
Burrowing anemones
peristalsis

24. Bauplan (polyp, Anthozoa)
Nervous system/ sensory structures

25. Bauplan (polyp, Anthozoa)
Nervous system/sensory structures
Mostly non-polar nerve cells
Bi-directional AP
Two-way synapses
No distinct dendrites
No ganglia or nerve cords
Nerve nets (2 arrays)
Polyps have minimal sensory structures
Mechanoreceptive hairs (also chemoreceptive?)
Ciliary cones (assoc. with cnidocytes)
Response to light

26. Bauplan (medusae, Hydrozoa)
Exumbrella
Subumbrella
Thick mesoglia
Elastic, aids in movement
Velum
Directs water jet
Increases water velocity
ring of tentacles
hollow, lined with gastrodermis

27. Bauplan (medusae, Hydrozoa)
Manubrium
Mouth & pharynx-like tube
Gastrovascular cavity
Four radial canals
ring canal
Tentacles
Digestion, etc… similar to polyps
No cnidocytes in g.v. cavity
Gonads outside of radial canals
sexes separate

28. Bauplan (medusae, Scyphozoa)
No velum
Mouth lobes with cnidocytes
Manubrium reduced or absent
Gastrovascular cavity
radial canals highly branched
Have cnidocytes in g.v. cavity
gastric filament: secretes enzymes
More effective extracellular digestion
gonads within g.v. cavity
sexes separate

29. Bauplan (medusae, Cubozoa)
Differences from hydromedusae
Gonads are within the g.v. cavity
Gastric filaments are present
Obvious quadriradiate symmetry

30. Nervous system/ sensory structures (medusae)
Overview
Neurons as in Anthozoa polyps
More organization and distinct sensory structures

31. Nervous system/ sensory structures (medusae)
Hydromedusae
Nerve net
Nerve rings (function?)
Statocysts (in some)
General sensory cells
Ocelli

32. Nervous system/ sensory structures (medusae)
Scyphomedusae
Nerve net
No nerve rings (usually)
Rhopalia:
Lower edge of bell
Include Statocysts, chemosensory pits and ocelli

33. Rhopalia: Scyphomedusae

34. Nervous system/ sensory structures (medusae)
Cubomedusae
Rhopalia
Up to 24 eyes
Cornea, lens and retina!
Up to 11,000 sensory cells per eye
Integration?
Images?
Function?