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?