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Biological Background
Jellyfishes belong to the phylum Cnidaria, a group of marine animals that have more than 9,000 species. Cnidarians have special stinging cells known as "cnidocytes," which they often use to catch prey. They are divided into three major classes: hydrozoa -- a group of sea ferns producing small jellies; scyphozoa -- the big jellies most often associated with jellies; and the anthozoa, or sea anemones.
The Umbrella
A jellyfish consists of three main external parts: the bell or umbrella; the tentacles; and numerous sensory organs situated all over its body. The umbrella is the major support system, allowing the jellyfish to keep its form. Its outer top layer consists of epidermal tissue known as the "ectoderm," while the inside layer is made up of a gelatin-like material, known as a "mesoglea." Beneath the mesoglea is another layer of tissue known as the "endoderm." The endoderm and the ectoderm serve as the mesoglea's bag that maintains a jelly's form.
The Sensory Organs
Since a jelly does not have any brain, it relies on sensory organs to control its body and understand its surroundings. Sensory organs detect light and ensure that the jelly's body is swimming upright or in the direction intended.
Sensory organs also command the muscles below the umbrella to contract, creating propulsion and allowing the jellyfish to move. Most species have sensory organs situated on the edges of the umbrella. Without sensory organs, the jellyfish will not be able to orient itself and have a stable support system.
Tentacles
A jellyfish mainly uses tentacles to gather food. Since jellies cannot move as fast as other animals, each tentacle is equipped with nematocysts or cnidocysts -- small organelles packed with toxins to immobilize prey. Jellies control their tentacles by muscle contraction, but they often let the ocean's current drift their tentacles to areas where prey is abundant. Once a tentacle stings a prey, it contracts to deliver the prey to the mouth.