Phylum Cnidaria
• Class Hydrozoa- hydra
• Class Scyphozoa- jellyfish
• Class Anthozoa- corals and sea anemones
Characteristics
• All aquatic, mostly marine
• Come in two basic forms
– Polyp- tubular, mouth and tentacles at one end, base
for attachment at the other end, sessile
– Medusa- expanded bell shape body, motile
Many Cnidarians exhibit both forms at some
point during their life cycle
Class Hydrozoa
• The hydra is our primary example
• Polyp
• As the hydra develops it forms to germ cell layers, the
ectoderm and the endoderm.
• Ectoderm forms the epidermis or outer skin
• The endodermis forms the gastrodermis or the cell
layer lining the gastrovascular cavity. Digestion occurs
in these cells
• Mesoglea separates the two cell layers
• Stays attached by its sticky basal disk
Germ layer
From Wikipedia
A germ layer, is a primary layer of cells that form during
embryogenesis. The three germ layers in vertebrates are
particularly pronounced; however, all eumetazoans,
(animals more complex than the sponge) produce two or
three primary germ layers. Animals with radial symmetry,
like cnidarians, produce two germ layers (the ectoderm
and endoderm) making them diploblastic. Animals with
bilateral symmetry produce a third layer (the mesoderm),
between these two layers. making them triploblastic.
Germ layers eventually give rise to all of an animal’s tissues
and organs through the process of organogenesis.
Embryology
Feeding
• Stinging cells (cnidoblasts) line the tentacles
and produce capsules containing poisonous
barbs, long coiled threads, or a sticky
substance (namatocysts).
• Namatacysts shoot out and grab prey which is
usually a worm or other small invertebrate.
Response
• Hydra is able to detect prey because it has a
network of nerve cells and fibers extending
throughout its body called a nerve net.
Reproduction
Asexual
• Budding
• Regeneration
Sexual
• Ova is produced in ovaries
• Sperm formed in testes
• Ovaries and testes can be seen as small swelling on the
hydras body wall
• Sperm are released and fertilize the ovum of another hydra.
This produces a zygote which divides and eventually breaks
off and continues to develop into a new hydra.
Class Scyphozoa
Phylum Cnidarian
Class Scyphozoa
Genus Aurelia (Common Jellyfish)
1. Sperm from male fertilizes egg in female gastrovascular
cavity
2. Zygote develops on oral arm and becomes a ciliated larva
called a planula
3. Planula swims away and attaches to base, now it’s in polyp
form and called a scyphistoma
4. Grows and develops several horizontal constrictions, it’s
called a strobila
5. Separate, each is an ephyra
6. Ephyra develops into large medusa
Class Anthozoa
• Corals and Sea Anemones
• “Flower animals”
• Corals- polyp living in self-made hard skeleton
made of calcium carbonate
• Sea anemones- large stout-bodied polyps with
rows of tentecles. Colorful because of
symbiotic algae
• The Great Barrier Reef off the coast of
Australia is the largest biological structure in
the world.
• Coral reefs are considered the rainforest of the
ocean meaning they are very diverse with lots
of different animals and species richness.