Although many differences exist between species that belong to the Cnidaria phylum, the presence of nematocysts or stinging cells unites this diverse group. While there are more than 10, 000 cnidarian species, some of the most familiar are jellyfish, sea anemones, coral and sea fans. Cnidarians mostly reside in the oceans although some species live in fresh water also.
Common Characteristics of Cnidarians
Cnidarians are radially symmetrical and have a U-shaped body with two layers of cells, an outer epidermis and an inner gastrodermis layer. A fibrous jelly called mesoglea fills the space between the layers. Cnidarians have a simple netlike nervous system. A single cavity serves for both food consumption as well as excretion of waste. Most cnidarians are carnivorous and kill by embedding their nematotocysts into their prey. They then release a toxin that paralyzes the prey. Some cnidarians go through their life cycle in both the polyp and medusa stages, while in others, one stage may dominate over the other or may even be absent altogether.
Types of Cnidarians
Anthozoa is the largest class of cnidarians with more than 6,000 species. The solitary sea anemones and the colonized corals come in this class. Anthozoas lack the medusa stage in their life cycle and live exclusively as polyps. Hydras, found in fresh water, are the most common species of the cnidarians class of Hydrozoa. Most hydrozoans live in colonies and go through both the polyp and medusa stages. True jellyfish come in the scyphozoan class of cnidarians. These solitary, free-floating cnidarians spend most of their life cycle in the medusa stage.
Jellyfish Vs Other Cnidarians
The main difference between jellyfish and other cnidarians is that the jellyfish live predominantly in the medusa stage, while the other cnidarians live in the polyp form.
The umbrella shaped jellyfish has its tentacles around the perimeter of its shape, while the other cnidarians are tubular and have their tentacles around the mouth. Polyp cnidarians have their mouths facing up, while the mouth of the jellyfish, a medusa, faces downward. Jellyfish have the ability to float, while corals and sea anemones are stationary. Jellyfish also have a more developed nervous system than other cnidarians that allows them to swim.
Other Jellyfish Facts
Jellyfish vary in size, from less than an inch to more than a foot in diameter. The colorful, mostly transparent jellyfish live both in shallow waters as well as deep down in the oceans. Jellyfish have eight sets of eye spots, making them sensitive to light. Although mostly solitary, some jellyfish may travel in groups of several thousand. While the sting of most jellyfish does not harm humans, some stings may cause cramps, fever or even death.
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