Scyphozoa

True jellies live in the sea.

They have a cup shape. They do not have a head or a bone skeleton. Instead, they have a soft body.

They use stinging parts to catch fish. Some jellies even eat tiny bits from the water. They swim by squeezing their muscles. This helps them move through the ocean. These jellies have lived in the sea for a very long time.

Scyphozoa are the true jellyfish. They live only in the ocean. These animals have lived on Earth since the Cambrian period.

Most jellyfish have two life stages. One stage is a tiny polyp. This polyp lives on the sea floor. The other stage is the medusa. This is the bell-shaped form we see in the water.

Jellyfish do not have bones or a head. Instead, they have mesoglea. This is a jelly-like material inside their bodies. It works like a skeleton to hold their shape. They swim by using muscles around the rim of their bell. They contract and relax these muscles to move. This helps them catch food or escape danger.

To eat, they use stinging cells called nematocysts. These cells are on their tentacles. Some jellyfish use them to catch fish or small shrimp. Other species are filter feeders. They use tentacles to strain tiny plankton from the water. Some jellyfish are very large. The species Cyanea capillata can grow quite big. Most jellyfish are mostly made of water.

Scyphozoa are known as the true jellyfish. They live only in the salt water of the ocean. These amazing animals have existed since the early Cambrian period.

The name Scyphozoa comes from a Greek word for a drinking cup. This name describes their cup-like shape. Most jellyfish are colorful and easy to see in coastal waters. They can be small or very large. The species Cyanea capillata can grow to be quite wide.

These creatures have a very interesting way they work. Most have two different life stages. First, they live as a tiny polyp on the sea floor. This polyp can live for a long time. Later, it gives rise to the medusa stage. The medusa is the bell-shaped animal we usually see. A polyp can make new medusae through a process called strobilation. This happens when the polyp buds off many tiny jellyfish from its top. The young medusae start out microscopic. They may take many years to grow up.

Jellyfish do not have a head or a hard skeleton. Instead, they use a jelly-like material called mesoglea. This material provides the structure they need to hold their shape. They move by using a ring of muscle fibers. These muscles are located around the rim of the bell. The jellyfish swims by contracting and relaxing these muscles. This movement helps them catch food or escape from predators. They also use stinging cells called nematocysts to eat. These cells are on their tentacles and oral arms. They use them to catch fish or small shrimp. Some jellyfish are filter feeders that strain plankton from the water.

Scientists have studied these animals for a long time. We know they have been around since the Cambrian era. Fossilized jellyfish have even been found at Blackberry Hill in Wisconsin. Most jellyfish are made of up to 98% water. Because they are so watery, they are rarely found as fossils. There are about 200 known species today. However, scientists think there are likely at least 400 species in the world. Some large species like Nemopilema nomurai live near Japan and China. These huge jellies can sometimes disrupt local fishing.

Learning about jellyfish helps us understand how life works in the sea. They have a simple nervous system made of a net of cells. Some have small structures called rhopalia to help them move. These lobes help coordinate their muscles. They also have sensory pits to help them feel their way. Even without a brain, they can react to the world. They show us how life can thrive with very simple parts. Their life cycle connects the sea floor to the open ocean.

Scyphozoa are a class of marine animals known as the true jellyfish. They belong to the phylum Cnidaria and live exclusively in saltwater oceans. These organisms are famous for their distinct cup-like shape. This shape is reflected in their name, which comes from the Greek word *skyphos*, meaning a drinking cup.

Scyphozoans have a very long history on Earth. Fossil evidence shows they have existed since the earliest Cambrian period. While they are often seen in coastal waters, they live throughout the world's oceans. They can be found anywhere from the surface to great depths. However, they never live in freshwater or on land.

The biology of a Scyphozoan involves a complex life cycle with two main phases. The first phase is a bottom-dwelling polyp. This stage is often inconspicuous and can live for a long time. The second phase is the planktonic medusa. The medusa is the bell-shaped form most people recognize.

The life cycle begins when a fertilized egg produces a planular larva. This larva quickly attaches to the sea floor and becomes a tiny, stationary polyp called a scyphistoma. The scyphistoma can reproduce asexually by budding new polyps. Eventually, the polyp undergoes strobilation. This is a process where the polyp buds off many tiny medusae from its upper surface. These young medusae start out microscopic and may take years to reach sexual maturity.

Anatomy in Scyphozoans is quite different from many other animals. They have no durable hard parts. This means they have no head, no skeleton, and no specialized organs for breathing or excretion. Instead, they rely on mesoglea for structural integrity. Mesoglea is an internal gelatinous material that acts like a skeleton. This material contains mobile amoeboid cells that come from the epidermis. Most Scyphozoans also display four-part symmetry. To move, they use a ring of muscle fibers located within the mesoglea around the rim of the dome. They swim by alternately contracting and relaxing these muscles. This rhythmic movement allows them to catch prey or escape from predators.

Feeding in Scyphozoans relies on specialized stinging cells called nematocysts. These cells are located on the tentacles that hang from the edge of the umbrella dome. They are also found on the four or eight oral arms that hang from the central mouth. Most Scyphozoans use these cells to capture crustaceans and fish. Some species are instead filter feeders. They use their tentacles to strain tiny plankton from the water. The mouth opens into a central stomach. From this stomach, four interconnected diverticula radiate outward. In many species, a system of radial canals helps distribute nutrients. Some genera, such as *Cassiopea*, even have smaller mouths located on their oral arms.

Even without a brain, Scyphozoans can sense their environment. Their nervous system is usually a distributed net of cells. Some species have more organized nerve rings. In species without these rings, nerve cells are concentrated into small structures called rhopalia. There are between four and sixteen of these lobes arranged around the rim of the umbrella. The rhopalia coordinate the muscular actions needed for swimming. Each rhopalium is typically connected to a pair of sensory pits. They also contain a statocyst and sometimes a pigment-cup ocellus. These parts help the animal stay oriented and detect light.

Scyphozoans vary greatly in size and impact. Most species range from a few centimeters to a few meters in diameter. The species *Cyanea capillata* can reach very large widths. Some jellyfish are commercially important, such as the moon jelly (*Aurelia aurita*). Others, like the enormous *Nemopilema nomurai*, can cause major disruptions to fisheries. This large species is found between Japan and China. Because jellyfish can consist of as much as 98% water, they are rarely preserved as fossils. However, we know they are diverse. While scientists recognize about 200 extant species, the true diversity is likely at least 400 species.

The classification of Scyphozoa has changed over time. They were once thought to include the classes Cubozoa and Staurozoa. Now, they include only three extant orders. Two of these orders are part of the subclass Discomedusae. The other major group is the subclass Coronamedusae. This complex system of life shows how simple structures can perform many vital tasks. From the sea floor polyp to the drifting medusa, Scyphozoans connect different parts of the ocean ecosystem.