3 Classes of Cnidarians

3 Classes of Cnidarians

• Cnidarians are sac-like animals that have radial symmetry, tentacles armed with stinging cells, two tissue layers, and no organs.

• Presently, they are grouped into three classes: 1. Hydrozoa2. Scyphozoa3. Anthozoa

CLASS HYDROZOA

Alternate between polyp and medusa forms.Freshwater and marine species.

Gonionemus vertens Clinging jellyfish

CLASS HYDROZOA

Numerous species, including Hydra, Obelia and Physalia

Hydra

Physalia

Obelia

CLASS HYDROZOA

Hydra is a solitary polyp inhabiting fresh water and found attached to the undersides of plant leaves and stems. It ranges in size from 1 to 4 cm.

Hydra oligactisBrown hydra

CLASS HYDROZOA

Obelia is a colonial organism composed of feeding polyps equipped with stinging cells and reproductive polyps that form the medusa stage.

CLASS HYDROZOA

The polyps are connected by hollow branches, which serve to distribute food to each individual polyp in the colony. Obelia is found along shallow coastal shores, on tops of rocks, and attached to pilings just below the waterline.

CLASS HYDROZOA

Physalia is another kind of colonial organism: It has long tentacles and uses a floating gas sac to move the colony of feeding and reproductive polyps across the ocean’s surface.

CLASS HYDROZOA

The Portuguese man-of-war is the most famous species from the genus. It can capture fairly large prey and its venom is powerful enough to kill a human

Physalia physalis

CLASS HYDROZOA

Zoologists call the hydrozoans that attach to some substrate and grow by branching tentacles. Colonial hydrozoans that floats are known as ciphonophores.

CLASS SCYPHOZOA Alternate between

polyp and medusa forms.

Jellyfish comprise the class.

All of the 200+ species are marine

Familiar examples include sea nettles and moon jellies

Chrysaora quinquecirrha

Aurelia aurita

CLASS SCYPHOZOA Sea nettles (Chrysaora quinquecirrha) are

common in the lower Chesapeake Bay in the summer months. They grow up to 18cm (7in). They come into the Bay to breed. By mid-summer, ripening gonads, shaped like a four-leaf clover, can be clearly seen though the medusa bell.

CLASS SCYPHOZOA

Mature sea nettles die after spawning. By late fall they are gone. Certain fish and crabs are resistant to their venom.

Moon jellyfish (Aurelia aurita) are also common in the lower Bay, but only in late summer. They can be as large as 30 cm (1 ft.).

Deadly Box Jellyfish

CLASS ANTHOZOA

Anthozoans, meaning “flower animals”, make up the third class of cnidarians and occur only in polyp form.

All anthozoans have a polyp body, which is topped with a tuft of hollow tentacles.

There are more than 6,000 species of anthozoans, and they are all marine animals.

CLASS ANTHOZOA

Members of this class include the sea anemones and corals.

CLASS ANTHOZOA

Sea anemones are found in all oceans of the world and come in a variety of colors. Species range in size from 1 cm to 1 meter. While they appear to be sedentary, they can glide, burrow, and somersault by bending over. Their diet consists of mostly of small fish and crustaceans and they are often found living on coral reefs.

CLASS ANTHOZOA

In the Chesapeake Bay, two kinds of sea anemones are found: the white or ghost anemone (Diadumene leucolena) and the green-striped anemone (Haliplanella luciae).

CLASS ANTHOZOA

Corals are small polyps surrounded and supported by a calcium-containing skeleton. Some species are solitary but most live in colonies.

Corals are grouped as either hard or soft corals.

CLASS ANTHOZOA

Hard corals, also known as true or stony corals, typically live in a colony of thousands of polyps united together. As members of the colony die, new polyps grow and unite on top of the skeleton of the old.

The massive stony structures we call reefs are formed by hard corals.

CLASS ANTHOZOA

Polyps of hard corals have 6 tentacles or multiples of six.

Soft corals are usually treelike and consist of polyps having eight tentacles. Sea fans are familiar soft corals.

Brain Coral

CLASS ANTHOZOA

Corals found in the Chesapeake Bay include the star coral (Astrangia astreiformis) and whip coral, or sea whip (Leptogorgia virgulata).

Phylum Cnidarian

Biology of Cnidarians