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

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

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

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

Overview Four classes Class Scyphozoa Large jellies Stauromedusae

Overview Four classes Class Cubozoa Box jellies www.zoologie-online.de/.../Cubozoa/cubozoa.html

Overview Four classes Class 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 Cnidocytes Functions Cnidae Focus: nematocyst-bearing cnidocyte Cell body Cnidocil: how triggered? Operculum

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

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

Overall body structure Cnidocytes Other 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 Acrorhagi: 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?