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Medusa (biology)

Medusa (biology)

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In biology
Biology
Biology is a natural science concerned with the study of life and living organisms, including their structure, function, growth, origin, evolution, distribution, and taxonomy. Biology is a vast subject containing many subdivisions, topics, and disciplines...

, a medusa (plural: medusae) is a form of cnidaria
Cnidaria
Cnidaria is a phylum containing over 9,000 species of animals found exclusively in aquatic and mostly marine environments. Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey. Their bodies consist of mesoglea, a non-living jelly-like substance,...

n in which the body is shaped like an umbrella, in contrast with polyp
Polyp
A polyp in zoology is one of two forms found in the phylum Cnidaria, the other being the medusa. Polyps are approximately cylindrical in shape and elongated at the axis of the body...

s. 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
Gastrovascular cavity
Gastro vascular cavity, as the name indicates, functions in both digestion and the distribution of nutrients and particles to all parts of the body...

 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 tentacle
Tentacle
A tentacle or bothrium is one of usually two or more elongated flexible organs present in animals, especially invertebrates. The term may also refer to the hairs of the leaves of some insectivorous plants. Usually, tentacles are used for feeding, feeling and grasping. Anatomically, they work like...

s as its said.

In the class Hydrozoa
Hydrozoa
Hydrozoa are a taxonomic class of very small, predatory animals which can be solitary or colonial and which mostly live in saltwater. A few genera within this class live in freshwater...

, medusae are the sexual individuals of many species, alternating in the life cycle with asexual polyps. The medusa form of Hydrozoans are known as hydromedusae.

The medusa form predominates in the classes Scyphozoa
Scyphozoa
Scyphozoa is a class within the phylum Cnidaria, sometimes referred to as the "true jellyfish".The class name Scyphozoa comes from the Greek word skyphos , denoting a kind of drinking cup and alluding to the cup shape of the organism....

 (the common, colorful, large jellyfish
Jellyfish
Jellyfish are free-swimming members of the phylum Cnidaria. Medusa is another word for jellyfish, and refers to any free-swimming jellyfish stages in the phylum Cnidaria...

) and Cubozoa. Except for freshwater hydrozoa
Hydrozoa
Hydrozoa are a taxonomic class of very small, predatory animals which can be solitary or colonial and which mostly live in saltwater. A few genera within this class live in freshwater...

ns like Craspedacusta sowerbyi
Craspedacusta sowerbyi
Craspedacusta sowerbyi is a freshwater jellyfish in the phylum Cnidaria. Since it is classified as a hydrozoan, it is one of many jellyfish that are also known as hydromedusae...

, these are the only classes in which medusae appear.

German biologist Ernst Haeckel
Ernst Haeckel
The "European War" became known as "The Great War", and it was not until 1920, in the book "The First World War 1914-1918" by Charles à Court Repington, that the term "First World War" was used as the official name for the conflict.-Research:...

 popularized medusae through his vivid illustrations, particularly in Kunstformen der Natur
Kunstformen der Natur
Kunstformen der Natur is a book of lithographic and autotype prints by German biologist Ernst Haeckel. Originally published in sets of ten between 1899 and 1904 and as a complete volume in 1904, it consists of 100 prints of various organisms, many of which were first described by Haeckel himself...

.