<P> In biology, a medusa (plural: medusae) is a form of cnidarian in which the body is shaped like an umbrella, in contrast with polyps . Medusae vary from bell - shaped to the shape of a thin disk, scarcely convex above and only slightly concave below . The upper or aboral surface is called the exumbrella and the lower surface is called the subumbrella; the mouth is located on the lower surface, which may be partially closed by a membrane extending inward from the margin (called the velum). The digestive cavity consists of the gastrovascular cavity and radiating canals which extend toward the margin; these canals may be simple or branching, and vary in number from few to many . The margin of the disk bears sensory organs and tentacles . </P> <P> German biologist Ernst Haeckel popularized medusae through his vivid illustrations, particularly in Kunstformen der Natur . </P> <P> Most jellyfish do not have specialized digestive, osmoregulatory, central nervous, respiratory, or circulatory systems . The manubrium is a stalk - like structure hanging down from the centre of the underside, often surrounded by oral arms, which connects with the mouth / anus at the base of the bell . This opens into the gastrovascular cavity, where digestion takes place and nutrients are absorbed . It is joined to the radial canals which extend to the margin of the bell, where tentacles are attached . Nematocysts, which deliver the sting, are located mostly on the tentacles; scyphozoans also have them around the mouth and stomach . Jellyfish do not need a respiratory system since their skin is thin enough that the body is oxygenated by diffusion . They have limited control over movement, but can use their hydrostatic skeleton to navigate through contraction - pulsations of the bell - like body; some species actively swim most of the time, while others are mostly passive . Depending on the species, the body contains between 95 and 98% water . Most of the umbrella mass is a gelatinous material--the jelly--called mesoglea which is surrounded by two layers of protective skin . The top layer is called the epidermis, and the inner layer is referred to as gastrodermis, which lines the gut . </P> <P> Jellyfish employ a loose network of nerves, located in the epidermis, which is called a "nerve net". Although traditionally thought not to have a central nervous system, nerve net concentration and ganglion - like structures could be considered to constitute one in most species . A jellyfish detects various stimuli including the touch of other animals via this nerve net, which then transmits impulses both throughout the nerve net and around a circular nerve ring, through the rhopalial lappet, located at the rim of the jellyfish body, to other nerve cells . </P>

Where are the stinging cells located on a jellyfish
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