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Terminology

The term medusa was coined by Linnaeus in 1752, alluding to the tentacled head of Medusa in Greek mythology.[3] This term refers exclusively to the non-polyp life-stage which occurs in many cnidarians, which is typified by a large pulsating gelatinous bell with long trailing tentacles. All medusa-producing species belong to the sub-phylum Medusozoa.
The English popular namejellyfish has been in use since 1796.[3] It has traditionally also been applied to other animals sharing a superficial resemblance, for example ctenophores (members from another phylum of common, gelatinous and generally transparent or translucent, free-swimming planktonic carnivores now known as comb jellies) were included as "jellyfishes".[4] Even some scientists include the phylum ctenophora whenthey are referring to jellyfish.[5] Other scientists prefer to use the more all-encompassing term gelatinous zooplankton, when referring to these, together with other soft-bodied animals in the water column.[6]
As jellyfish are not even vertebrates, let alone true fish, the usual word jellyfish is considered by some to be a misnomer, and American public aquariums have attempted to popularize use ofthe terms jellies or sea jellies instead.[7]
Many textbooks and websites refer to only scyphozoans as "true jellyfish".[8][9]
A group of jellyfish is sometimes called a bloom or a swarm.[10] "Bloom" is usually used for a large group of jellyfish that gather in a small area, but may also have a time component, referring to seasonal increases, or numbers beyond what was expected.[11] Anothercollective name for a group of jellyfish is a smack,[12] although this term is not commonly used by scientists who study jellyfish. Jellyfish are "bloomy" by nature of their life cycles, being produced by their benthic polyps usually in the spring when sunshine and plankton increase, so they appear rather suddenly and often in large numbers, even when an ecosystem is in balance.[13] Using "swarm"usually implies some kind of active ability to stay together, which a few species such as Aurelia, the moon jelly, demonstrate.[14]
Medusa jellyfish may be classified as scyphomedusae ("true" jellyfish), stauromedusae (stalked jellyfish), cubomedusae (box jellyfish), or hydromedusae, according to which clade their species belongs.[15]
In biology, a medusa (plural: medusae) is a form of cnidarian inwhich 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 themargin (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 as its said.
German biologist Ernst Haeckel popularized medusae through his vivid illustrations, particularly in Kunstformen derNatur.
Anatomy



The major surfaces and axes of a jellyfish
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, with the mouth at its tip. 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.[16] 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...