Anatomy of a Jelly Fish

All mature jellyfish subscribe to a shape that is properly called "radial symmetry," meaning the parts of their bodies all radiate from a central axis. All species have a bell -- the usually-transparent body -- and tentacles pendant from the bell. A jellyfish is made of more than 96-percent water. Members of their phylum, Cnidaria, thrive in every ocean, some living on the surface and others living in the icy blackness 12,000 feet below.

  1. Anatomical Reference Terms

    • Jellyfish have four distinct parts: the oral surface below the bell; the aboral surface above the bell; the proximal end of the tentacles where they join the body; and the distal end at the tip of the longest tentacles. Two measurements are used in describing jellyfish: the oral-aboral axis describes the length between the crown of the aboral surface and the base of the proximal end, while the proximo-distal axis is a measurement of the distance between the proximal end and the distal end.

    Comparative Anatomy

    • Jellyfish do not have stomachs, intestines, a liver or pancreas; they digest food inside a cavity that has only one lining. They do not have a respiratory system of any kind, but they absorb what oxygen they need through their skin. They lack a brain and respond automatically to basic stimuli such as the presence of food and light. Jellyfish also have a loose net of nerves draped around their exterior that communicate directly with other nerves. They move by flexing their bell-shaped body. Because they have no eyes, the jellyfish must use light-sensitive organs that define up from down in terms of lighter and darker areas.

    The Bell

    • The bell -- sometimes called the hood -- is that mass which cab be called the animal's body. It is a mass of jelly, properly called mesoglea, held in place by two layers of skin cells. One layer is above the jelly, and it is called the umbrella; the other layer is below the jelly and is called the subumbrella. The whole skin cell layer is properly called the epidermis, and it covers the entire external surface of the body. Inside the mesoglea is an inner layer of cells called the gastrodermis, which surrounds the digestive cavity, properly called the coelenteron. The coelenteron is gullet, stomach and intestine all in one, and has only a single opening that corresponds to both the mouth and anus in more complex animals.

    The Tentacles

    • Between four and eight small tentacles -- the number depends upon species, not age or injury -- surround the coelenteron. These appendages are properly called oral arms. The oral arms aim food into the coelenteron. The food is initially captured by the larger tentacles that are pendant from the bell. Each tentacle is armed with venomous cells called cnidocytes, used to trap and kill prey and for defense; it is these cells that can sting humans who come into accidental contact.

    Body Size and Tentacle Length

    • Most freshwater jellyfish, and some marine species, are less than an inch across the bell. The normal size for a mature marine specimen is approximately one foot across the bell. The largest marine specimens grow to more than 7 feet across. Tentacles of the largest specimens often grow to be more than 100 feet long.