1. Explain the classification and the phylogeny of each phyla.

Scientists classify cnidarians into three classes:

Hydrozoa include polyps, medusa, and species that alternate between the two

•Scyphozoa, the true jellyfishes, spend most of the life cycle as medusa

•Anthozoa live only as polyps and include anemones and corals

Phylum Cnidaria( also called Coelenterata): usually free-swimming, baglike body of two cell layers with a noncellular substance between them; gastrovascular cavity with one opening leading to the outside; many with tentacles and all with stinging capsules; solitary or colonial forms; marine and freshwater; hydroids, jellyfish, corals.

Class Hydrozoa : solitary or colonial; freshwater and marine; reproduction by asexual buds and gametes; alternation of generations in many: hydra, physalia.

Class Scyphozoa : Exclusively marine; most have mesentaries; polyp stage usually absent; jellyfish.

Class Anthozoa : Marine forms; solitary or colonial; numerous tentacles: sea anemones, corals, sea fans.  

2. Describe the characteristics typical for the diversity shown by each phyla

Tiny freshwater hydra, stinging jellyfish, and flowerlike coral all belong to the Phylum Cnidaria. The name of the phylum comes from cnidocytes, the stinging cells that characterize these organisms. The members of the phylum are called cnidarians. They are also known as coelenterates, a name that comes from their coelenteron, or "hollow gut." In addition to their stinging cells and hollow guts, cnidarians also have flexible structures called tentacles.

The body of a cnidarian may be either vase shaped or bell shaped. Scientists refer to the vase-shaped form as the polyp, and to the bell-shaped form as the medusa. The polyp is specialized for a sessile existence, the medusa for swimming. During their life cycles some cnidarians go throughout both polyp and medusa stages. Jellyfish, for example, go through both stages but spend most of their lives as medusa. Other cnidarians live only as polyps, and still others live as mixed colonies of polyps and medusa.

As either polyp or medusa, the organism consists of two cell layers - the endoderm and the ectoderm. Between these two layers is a jelly-like material called mesoglea.

The relationship between the structure and function of the cnidarians’ body parts is clearly seen in the way they feed. The tentacles capture the pray and paralyze it with coiled stingers called nematocysts, located inside the cnidocytes. The tentacles then draw the prey to the mouth. Enzymes inside the gastrovascular cavity break up the prey, and the specialized cells lining the cavity absorb the food. Undigested food and waste are expelled through the mouth.

Scientists classify cnidarians into three classes:

•Hydrozoa include polyps, medusa, and species that alternate between the two

•Scyphozoa, the true jellyfishes, spend most of the life cycle as medusa

•Anthozoa live only as polyps and include anemones and corals

The class Hydrozoa includes about 3,700 species, most of which live in oceans. However, the freshwater species that make up the genus Hydra have been extensively studied. Hydras range in size from only 1 to 4 cm. Most hydras are white or brown, but some appear green because of algae living beneath their outer cells. Hydras live in quiet ponds, lakes and streams. They attach themselves to rocks or water plants by means of sticky secretion produced by a group of cells called the basal disk.

Sometimes hydras leave one place of attachment and move to another. The basal disk can secrete bubbles of gas, which cause the hydra to float upside down on the surface. The hydra can also move by somersaulting. This particular movement occurs when the tentacles and the mouth end bend over and touch the bottom of the pond while the basal disk pulls free. This and other kinds of movement are made possible by the nerve net, interconnecting nerve cells that are located in the mesoglea.

Hydras capture pray with nematocysts. They wait for small floating animals to drift past their tentacles. Just a touch triggers a barrage of paralyzing nematocyst barbs, each on a thin thread that entangles the pray. The nerve net signals the tentacles to push the pray through the mouth and into the gut. This space is lined with specialized gastrodermal cells. Some of these cells secrete enzymes that begin the digestion of the food. Others take in the partially digested food and complete digestion.

Hydras generally reproduce asexually during warm weather. In asexual reproduction small buds form on the outside of the hydra’s body. These buds grow their own tentacles and then separate from the body and begin living independently.

Sexual reproduction usually occurs in the fall, when low temperatures triggers the development of eggs and sperm. The eggs are called ovaries. Motile sperm are formed by meiosis is similar swellings called testes. In some species eggs and sperm are produced in the same hermaphroditic individual. In others the individuals are either male or female. In either case sperm are released into the water. Some will reach the ovary of a nearby hydra, and one sperm will fertilize an egg cell. The fertilized egg, or zygote, then divides and grows into a ball of cells with a hard cover. The cover is an adaptation that protects the embryo throughout the winter until it develops into a new hydra in the spring.

Hydras are definitely not typical of the class. Hydras live independently; most other hydrozoas live in colonies. Although hydras exists only in polyp form, most other hydrozoas go through a medusa stage. The genus Obelia is a colonial hydrozoas that has many polyps attached to branched stalks. Some branches are specialized for feeding, while others are specialized for reproduction.

Perhaps the most remarkable hydrozoas is the Portuguese man-of-war, a member of the genus Physalia, which exists as a colony of modified polyps and medusa. Tentacles up to several meters long dangle from a large medusa that is specialized as a gas-filled float. The float can measure as much as 30 cm across. The tentacles bear polyps specialized for feeding, digestion, and sexual reproduction. The Portuguese men-of-war preys mostly on small fish, but its nematocysts contain a poison that can be harmful and even fetal to humans.

The name Scyphozoa means "cup animals", the dominant form of the life cycle of this class, the medusa. Scyphozoans are known commonly as jellyfish, which are classified into more than 200 species. Some have cups or bells measuring as little as 2 cm across or as much as 4 m. Several species of jellyfish trail tentacles many meters long below them in the water. The nematocysts of some jellyfish carry poisons that can cause severe pain and even death. Sea wasp jellyfish have killed several dozen people in the seawaters along the northern coast of Australia.

The common jellyfish Aurelia is a good example of a scyphozoan whose life cycle includes both medusa and polyp forms. Adult medusa release sperm and eggs into the water, where fertilization occurs. The resulting zygote develops into a blastula. The blastula then develops into a ciliated larva called a planula, which settles and attaches to the ocean bottom. The unattached end develops a mouth and tentacles, becoming a polyp. As the polyp grows, it forms stacks of medusa, each of which buds off and develops into a jellyfish.

The word Anthozoa means "flower animals", a fitting description for the approximately 6100 species of this class. These brightly colored marine cnidarians include sea anemones and corals.

Sea anemones are polyps commonly found in coastal areas. They attach themselves to rocks and other submerged objects by means of a basal disk. Anemones feed on fishes and other kinds of marine life that swim within the reach of their tentacles. Some sea anemones have a symbiotic relationship with the clown fish. The fish hides in the anemone’s tentacles, protected by a special chemical that prevents the nematocysts from firing. The two animals share food and protect each other from various predators. The movements of the fish also help prevent sediments from harming the anemone.

Corals are small polyps, most of which live in colonies. They built rocklike reefs by cementing their calcium based skeletons to those of adjoining polyps. When the polyps die, their hardened skeletons remain, serving as the foundation for new coral polyps. Thousands of coral polyps live together in this manner, building up a large and colorful reef communities. The reef provides food and shelter for an enormous variety of fishes and invertebrates.

Corals built three kinds of reefs. Fringing reefs form close to a beach, either on the mainland or on an island. Barrier reefs form farther out. One theory holds that barrier reefs were once fringing reefs that became further separated from the mainland or island when coastal lands sank. Attols, reefs with open lagoons in the middle, form far out at sea. Many are attached to the tops of undersea mountains or extinct volcanoes.

Coral reefs form only in the warm, clear waters at shallow depths. These conditions are required to allow photosynthesis to be carried out by certain algae that live symbiotically inside corral cells. The corals depend upon the algae as a source of oxygen and also as a means of speeding up the accumulation of calcium. The algae in turn depend upon the corals to supply vital nutrients. This symbiotic relationship is yet another example of how invertebrates have adapted to a sessile existence in the oceans.

3. List all the animals which are used as examples.

Class Hydrozoa. Hydrozoas. Hydra, Obelia, Gonionemus, Physalia

Class Cubozoa. Sea wasps. Tripedalia

Class Scyphozoa. Jellyfish. Aurelia, Pelagia, Cyanea

Class Anthozoa. Sea anemones and corals. Metridium, Pennatula, Gorgonia, Astrangia

5. Make a vocabulary list and briefly define them. Use all words printed in bold print.

1. Cnidocytes - stinging cells characterizing organisms in phylum Canidaria

2. Cnidarians - The members of the phylum cnidaria

3. Coelenterates - another name for the members of the phylum cnidaria that comes from their coelenteron, or "hollow gut".

4. Tentacles - flexible structures that cnidarians have to capture their prey

5. Polyp - vase-shaped form of a cnidarian, specialized for sessile existence

6. Medusa - bell-shaped form of a cnidarian, specialized for swimming

7. Mesoglea - either medusa or polyp consist of two cell layers - the endoderm and ectoderm, the mesoglea is a jellylike material between these two cell layers.

8. Nematocysts - coiled stingers located inside the cnidocytes used to paralyze pray

9. Gastrovascular cavity - The inside of a cnidarian, where the prey is broken up by enzymes

10. Ovaries - the body swellings on hydras where eggs are produced by meiosis

11. Testes - the body swelling on hydras where motile sperm are produced meiosis

12. Planula - when a zygote develops into a blastula, the blastula then develops into a ciliated larva called a planula, which settles and attaches to the ocean bottom.

6. List the adaptations which make this phylum successful on earth.

Tiny freshwater hydra, stinging jellyfish, and flowerlike coral all belong to the Phylum Cnidaria. The name of the phylum comes from cnidocytes, the stinging cells that characterize these organisms. The members of the phylum are called cnidarians. They are also known as coelenterates, a name that comes from their coelenteron, or "hollow gut." In addition to their stinging cells and hollow guts, cnidarians also have flexible structures called tentacles.

The body of a cnidarian may be either vase shaped or bell shaped. Scientists refer to the vase-shaped form as the polyp, and to the bell-shaped form as the medusa. The polyp is specialized for a sessile existence, the medusa for swimming. During their life cycles some cnidarians go throughout both polyp and medusa stages. Jellyfish, for example, go through both stages but spend most of their lives as medusa. Other cnidarians live only as polyps, and still others live as mixed colonies of polyps and medusa.

As either polyp or medusa, the organism consists of two cell layers - the endoderm and the ectoderm. Between these two layers is a jelly-like material called mesoglea.

The relationship between the structure and function of the cnidarians’ body parts is clearly seen in the way they feed. The tentacles capture the pray and paralyze it with coiled stingers called nematocysts, located inside the cnidocytes. The tentacles then draw the prey to the mouth. Enzymes inside the gastrovascular cavity break up the prey, and the specialized cells lining the cavity absorb the food. Undigested food and waste are expelled through the mouth.