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Invertebrate Zoology
Lecture 6: Phylum Cnidaria, Part 1
Lecture outline
Overview Overall body structure
Body wallCnidocytes
Bauplan, polyp (Anthozoa) Bauplans, medusae
HydrozoaScyphozoaCubozoaFocus on nervous systems/sensory structures
Overview
Thought to be plants until 18th century Two basic body forms diverse array of
organisms
Overview
Four classesClass Hydrozoa
[Clockwise from top]
Hydroids Hydromedusae by-the-wind
sailors Siphonophores
Siphonophore Photo: K. Raskoff / Monterey Peninsula College
Overview
Four classesClass Scyphozoa
Large jellies Stauromedusae
Overview
Four classesClass Cubozoa
Box jellies
www.zoologie-online.de/.../Cubozoa/cubozoa.html
Overview
Four classesClass Anthozoa
Anemones Coral Sea pansies Gorgonians
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?
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
Overall body structure
Body wall Epidermis
Epitheliomuscular cells: Contractile
portion at base (=myoneme)
Epidermal gland cells
Sensory cells Nerve cells Cnidocytes
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
Overall body structure
CnidocytesFunctionsCnidaeFocus: nematocyst-
bearing cnidocyte Cell body Cnidocil: how
triggered? Operculum
Overall body structure
CnidocytesFocus: nematocyst-
bearing cnidocyte Nematocyst capsule
With toxins Shaft Stylets and spines
Function? Hollow filament
Function?
Overall body structure
Cnidocyte: firingTrigger cnidocilEversion of shaft and
filamentInjection of toxins
HypothesesOsmotic hypothesisTension hypothesisContractile hypothesis
Overall body structure
CnidocytesOther cnidae (adhesive)
Spirocysts Ptychocysts (no image)
Bauplan (polyp, Anthozoa)
Bauplan (polyp, Anthozoa) Tentacles
Not true appendages (why not) Contain branches of the g.v. cavity Epidermal layer with cnidocytes AcrorhagiAcrorhagi: specialized defensive tentacles
Bauplan (polyp, Anthozoa)
Oral disc/ mouth/ pharynx
Epidermis extends down pharynx NOTE: Hydrozoan polyps
lack a pharynx.
Bauplan (polyp, Anthozoa)
Siphonoglyph Ciliated grooves:
create water current Often have two
Water flow pattern? Reversal of flow?
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
Bauplan (polyp, Anthozoa) Gastrovascular cavity
Mesenteries (x-sec) Cnidocytes associated with mesenteric filaments.
Acontia: Free-hanging filaments; defensive Gonads associated with mesenteries (Anthozoa) Myonemes
Bauplan (polyp, Anthozoa)
Column/Pedal disc Attachment
Epidermal gland cells
Movement Why move? Sessile polyps
Bauplan (polyp, Anthozoa) Column/Pedal disc
Movement (cont.) Burrowing anemones
peristalsis
Bauplan (polyp, Anthozoa)
Nervous system/ sensory structures
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
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
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
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
Bauplan (medusae, Cubozoa) Differences from hydromedusae
Gonads are within the g.v. cavity Gastric filaments are present
Obvious quadriradiate symmetry
Nervous system/sensory structures (medusae)
Overview Neurons as in
Anthozoa polyps More organization
and distinct sensory structures
Nervous system/sensory structures (medusae)
Hydromedusae Nerve net Nerve rings (function?) Statocysts (in some) General sensory cells Ocelli
Nervous system/sensory structures (medusae)
Scyphomedusae Nerve net No nerve rings
(usually) Rhopalia:
Lower edge of bell Include Statocysts,
chemosensory pits and ocelli
Rhopalia: Scyphomedusae
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?