Phylum Cnidaria

Classes: We are only going to talk about 4 classes

Hydrozoa: Portuguese Man-of-War, hydra, Obelia

Scyphozoa: True Jellyfish

Cubozoa: Box Jellyfish

Anthozoa: Sea anemones, Coral, and Sea fans

Hydrozoa• 2700 species

• Two groups:

• Hydroids

• Siphonophores

• Mainly marine animals

• Two life stages

• Polyp (hydra’s only have polyp stage)

• Medusa

• Found in mainly in polyp colonies

Scyphozoa• 200 Species

• True Jellyfish

• Marine coastal waters

• Two stages

• Polyp (restricted to small larval forms)

• Medusa

Cubozoa• 20 species

• Marine coastal water

• Two Stages

• Polyp

• Medusa (dominate stage)

• Square shaped when viewed from above

Anthozoa• 6200 species

• Marine coastal waters

• One stage

• Polyp only

• Most live in colonies

• Coral Reef

Characteristics

Radial Symmetry

Characteristics One body opening

Two cell layers are organized into tissues with specific functions.

The inner layer is adapted mainly to assist in digestion.

Oxygen dissolved in water can diffuse directly into body cells.

Carbon dioxide and other wastes can move out of a cnidarian’s body cells directly into the surrounding water

Gastrodermis

Characteristics

Two stages• Polyp

• Medusa

One body form may be more observable than the other.

In jellyfishes, the medusa is the body form usually observed.

The polyp is the familiar body form of hydras

Life Cycle & Reproductio

n

Medusa only life cycle Polyp only life

cycle

Life Cycle & Reproductio

nAll cnidarians have the

ability to reproduce sexually and asexually.

Sexual reproduction occurs in only one phase

of the life cycle

It usually occurs in the medusa stage, unless there

is no medusa stage then the polyp can reproduce

sexually

Asexual Reproduction

Sexual Reproduction

spermegg

Digestion

• Cnidarians are predators that capture their prey using nematocysts

• A nematocyst is a capsule that contains a coiled threadlike tube

• May be sticky or barbed, and it contains toxic substances

• Nematocysts are located in stinging cells called cnidocytes, on the tentacles

Digestion

Once captures by nematocysts, prey is brought to the mouth by contraction of the tentacles

Prey is moved to the gastrovascular cavity

The cells that line the gastrovascular cavity release enzymes over captured prey. Any undigested materials are ejected back out through the mouth.

Hydra eating a daphnia

Nervous system A cnidarian has a simple nervous system without a control center, such

as a brain like that of other animals.

In cnidarians, the nervous system consists of a nerve net that conducts impulses to and from all parts of the body

The impulses from the nerve net cause contractions of muscle-like cells in the two cell layers.