Cnidarians have two body forms—polyp and medusa—which often occur within the life cycle of a single cnidarian. Sea nettle (Chrysaora fuscescens). The body of a medusa, commonly called a jellyfish, usually has the shape of a bell or an umbrella, with tentacles hanging downward at the margin.

There are two basic cnidarian body shapes: a polyp form, which is attached to a surface; and an upside-down free-floating form called a medusa.

Cnidocytes, also known as stinging cells, are specialized neural cells that typify the phylum Cnidaria (sea anemones, corals, hydroids, and jellyfish) [1,2,3]. These cells contain an organelle called cnida or cnidocyst, which is the product of extensive Golgi secretions.

Cnidarian body forms. Cnidarians have two distinct body plans, the medusa (a) and the polyp (b).

Cnidarians have two distinct body plans, the medusa (a) and the polyp (b). All cnidarians have two membrane layers, with a jelly-like mesoglea between them. Animals in this phylum display two distinct morphological body plans: polyp or “stalk” and medusa or “bell” (Figure 2).

The gastrodermis is the inner layer of cells that serves as a lining membrane of the gastrovascular cavity of Cnidarians. The term is also used for the analogous inner epithelial layer of Ctenophores. It has been shown that the gastrodermis is among the sites where early signals of heat stress are expressed in corals.

Nematocysts are the means by which coelenterates capture prey and defend against predation. The 25 or more known types of nematocysts can be divided into to four functional categories: those that pierce, ensnare, or adhere to prey, and those that adhere to the substrate.

The Phylum Cnidaria consists of sessile or free swimming animals, characterised by primary radial, often modified as biradial or quadriradial, symmetry; and provided with unique cnidae (stinging capsules). The basic structure is sac-like, with a single terminal opening, functioning both as mouth and anus.

Nematocysts contain coiled threads that may bear barbs. The outer wall of the cell has hairlike projections called cnidocils, which are sensitive to touch. When touched, the cells are known to fire coiled threads that can either penetrate the flesh of the prey or predators of cnidarians (see Figure 1) or ensnare it.

The name Cnidaria comes from the Greek word “cnidos,” which means stinging nettle. Casually touching many cnidarians will make it clear how they got their name when their nematocysts eject barbed threads tipped with poison.

The five main characteristics of cnidarians are:

• Radial symmetry.
• Diploblastic animals.
• Tissue level of organisation.
• Presence of cnidoblasts with stinging nematocysts on the tentacles.
• Polymorphism and have two body forms, i.e. polyp and medusa.

The most notable is that instead of tentacles armed with stinging cells, comb jellies have sticky cells called colloblasts that do not sting and eight rows of cilia, or combs, that propel them through the water and produce a flickering rainbow-like appearance with their movements.

Most aren’t lethal, but a few are: some species, including the box jellyfish (most commonly found in and near Australia), can deliver a sting strong enough to kill a human in just a few minutes. If you’re in an area where it is known that jellyfish like to hang out, skip the swim altogether.

Comb jellies aren’t harmful to humans, but they wreak havoc on the local ecosystem. In the Adriatic Sea, they don’t have any predators yet. The rapidly reproducing comb jellies deplete supplies of plankton, as well as the eggs and larvae of fish like anchovies.

Jellyfish are usually from 1 inch to 16 inches in size (some like the lion’s mane can get up to 6 ft across with 50 ft tentacles!) while comb jellies are smaller, usually no more than 1-2 inches long. Jellyfish usually consumer larger prey, including comb jellies. They are one of the major predators for comb jellies.

Turritopsis dohrnii

The flower hat jelly (‘Olindias formosus’) is a species of hydrozoan in the family ‘Olindiidae’. Although they look like a jellyfish, they actually belong in the class ‘Hydrozoa’, while true jellyfish belong in class Scyphozoa.

scyphozoans

You can eat jellyfish in many ways, including shredded or sliced thinly and tossed with sugar, soy sauce, oil, and vinegar for a salad. It can also be cut into noodles, boiled, and served mixed with vegetables or meat. Prepared jellyfish has a delicate flavor and surprisingly crunchy texture.

Common signs and symptoms of jellyfish stings include: Burning, prickling, stinging pain. Red, brown or purplish tracks on the skin — a “print” of the tentacles’ contact with your skin. Itching.

Unfortunately, in the real world treating a jellyfish sting by urinating on it may actually cause someone in Monica’s situation even more pain, rather than relief. Urine can actually aggravate the jellyfish’s stingers into releasing more venom. This cure is, indeed, fiction.

What If You Get Stung By a Jellyfish?

1. Rinse the area with vinegar. (Not cool fresh water or seawater, which could make it worse.)
2. Avoid rubbing the area, which also can make things worse.
3. Use tweezers to pull off any tentacles still on your skin.
4. Do not put ice or ice packs on a sting.
5. Check with your doctor.

Prevention

1. Get information about conditions. Talk to lifeguards, local residents or officials with a local health department before swimming or diving in coastal waters, especially in areas where jellyfish are common.
2. Avoid water during jellyfish season.
3. Use protective lotions.
4. Wear a protective suit.