Jellyfish are marine invertebrates with bell-shaped bodies and stinging tentacles. Because jellyfish are not fish, scientists refer to them as scyphozoans, the "true jellyfish". They are more than 90% water and have no heart, bones or brain. Some swim by jet propulsion, but most are carried by the ocean tides and currents. Jellyfish belong to the cnidarian phylum of invertebrates, which also includes sea anemones, sea pens, hydrozoans and coral polyps.

Body parts

Jellyfish come in many shapes and sizes, but they share some key features. Their body, known as a bell, consists mostly of a jelly-like substance contained inside a thin covering of tissue. Hanging down from the bell are long, thin tentacles lined with special cells called cnidocytes. These fire tiny "harpoons", called nematocysts, containing venom at great speed at its prey (we humans feel these as stings). The nematocysts are used mainly to capture prey by paralysing it before certain tentacles known as oral arms draw it into the creature's mouth, usually found on the underside of the bell. This is also the jellyfish's anus. Some jellies have neither mouths nor tentacles, and take in food directly through openings in their oral arms.

Jellyfish feed on small fish and zooplankton that become caught in their tentacles. As they drift in the water, jellyfish rhythmically open and close their bell-like body. This action creates a current which forces prey to within reach of their tentacles.

Lion's mane jellyfish

The lion's mane jellyfish is the largest known jellyfish. It lives in the cold waters of the Arctic, northern Atlantic and northern Pacific oceans. It is common in British and Irish waters, as well as around Scandinavia. The largest recorded specimen ever found had a bell measuring 2.3 metres (7 ft 6 in) across and tentacles 37 metres (120 feet) long, making it longer than the blue whale. Most individuals are much smaller.

​​​​​​​The lion's mane jellyfish’s sticky tentacles are grouped into eight clusters, each cluster containing over 100 tentacles. It uses them to prey on fish and other smaller jellyfish. It is itself preyed on by seabirds, large fish such as the ocean sunfish and sea turtles.

Siphonophores

Siphonophores (their name means "tube bearing") form another group of cnidarians called hydrozoans. Although a siphonophore is an individual organism, it is a colony of much smaller organisms called zooids. These zooids work together to carry out the functions of a single animal. Some zooids are responsible for reproduction, some for digestion, some keep the colony buoyant in the water while others move it along by jet propulsion. Many siphonophores are long, thin and transparent.

​​​​​​​Nearly all siphonophores are bioluminescent, emitting light both to attract and attack prey. Their diets consist of small crustaceans called copepods and small fish. They use nematocysts, stinging capsules on their tentacles, to paralyse their prey before devouring it.

Portuguese 
​​​​​​​man-o'-war

Commonly mistaken for a jellyfish, the Portuguese man-o'-war is, in fact, a siphonophore. Found in the Atlantic Ocean and the Indian Ocean, it is regarded as the same species as the Pacific man-o'-war or blue bottle, found in the Pacific.

Like all siphonophores, the Portuguese man-o'-war is a colony of smaller organisms called zooids. Stinging tentacle zooids catch food, feeding zooids digest it and one large inflated zooid keeps the colony afloat. The upper part of the man-o'-war  is like a sac of air. It floats on the surface, dragging its tentacles below the water, trapping small fish. Lining its tentacles are venomous nematocysts. These are powerful enough to kill fish, and have been known to occasionally kill humans.