Cnidaria
Dissection Guide
Slide study guide
Cnidaria
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Diploblastic; endoderm & ectoderm separated by
mesoglea
Radial symmetry
Presence of cnidae/nematocysts in cnidocytes (stinging
cells) (see photo at right)
Alternation of polyploid (polyp) and medusoid generations
Gastrovascular cavity with single mouth/anus
Tissue grade organization
Additional Classes not represented in lab:
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Cubozoa: box jellies and sea wasps.
Staurozoa: stalked jellyfish.
Myxozoa: spore-producing parasites
Class Hydrozoa — Hydroids
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Alternation of generations: polypoid generation
usually dominant
Medusae usually small, transparent
A few (fire corals) produce coral-like calcareous
skeleton
Polyps usually colonial, with interconnected
coelenterons
No cells in mesoglea
Nematocysts only in epidermis (not gastrodermis)
Marine and fresh waters
Hydra
Hydra
Hydra with developing ovaries
Hydra with ovaries
and buds
Hydra with developing testes
(spermaries)
Hydra Nerve
Net
neurite
neuron
Obelia
Class Anthozoa —
Anemones & Corals
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Lack medusoid stage
Polyps solitary or colonial
Coelenteron (gastrovascular cavity) subdivided by
longitudinal folds of tissue
Tentacles in multiples of 6 (Hexacorallia) or 8
(Octocorallia)
Marine only
Subclass Hexacorallia (Zoantharia)
Order Actiniaria
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Anemones
Polyps large, usually
with distinct stalk
Numerous tentacles
Lack calcified skeleton
Solitary or in
aggregations but not
truly colonies
acontia
Oral surface of the sea anemone
showing the tentacles, mouth and
siphonoglyphs.
Cross section through a Metridium
looking towards the oral opening shows complete and incomplete
septa.
Detail of the oral opening of the sea
anemone showing the position of the
siphonoglyphs.
Internal anatomy of the sea anemone
- showing the body wall
Cross section through the middle of a
sea anemone- Shows complete and
incomplete septa.
Cross section through a Metridium
looking towards the basal disc - shows
complete and incomplete septa.
Longitudinal section of a sea anemone
showing the internal anatomy
Internal anatomy of the sea anemone showing the pharynx and the acontia
attached to the base of the cavity
Internal anatomy of the sea anemone
showing the acontia
Mesoglea
siphonoglyphs
Metridium- Cross section of the anemone,
slide, showing the body wall incomplete
septa and trilobed ends.
Order Scleractinia
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Stony corals
Secrete calcareous
skeleton
Most colonial with small
polyps, though some
solitary
But some solitary with
large polyps
Order Alcyonacea
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Soft corals
Lack rigid skeleton
Support provided by
calcareous spicules
Colonies often tree-like
or resemble anemone
Order Gorgonacea
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Sea whips and sea fans
Small polyps form
branching
Skeleton usually
proteinaceous
Sometimes included
within the Alcynonacea
colonies
Class Scyphozoa —
Jellyfish
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Medusoid stage
predominates; polyps
inconspicuous
Thick gelatin-like
mesoglea
Often pigmented
Aurelia
Rhopalium of Aurelia
Rhopalium of Aurelia
Rhopalium
Aboral surface is concave and resembles a large
sucker. This suction assists the animal in maintaining
contact with the substratum. With it the jellyfish can
remain in place on smooth vertical surfaces as you
have observed in the aquarium.