Box jellyfish are 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 invertebrates distinguished by their cube

Cube

In geometry, a cube is a three-dimensional solid object bounded by six square faces, facets or sides, with three meeting at each vertex. The cube can also be called a regular hexahedron and is one of the five Platonic solids. It is a special kind of square prism, of rectangular parallelepiped and...

-shaped medusae

Medusa (biology)

In biology, a medusa 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...

. Box jellyfish are known for the extremely potent venom produced by some species: Chironex fleckeri

Chironex fleckeri

Chironex fleckeri, commonly known as sea wasp, is a species of Box jellyfish found in coastal waters from northern Australia and New Guinea north to the Philippines and Vietnam. It has been described as "the most lethal jellyfish in the world", with at least 63 known deaths from 1884 to...

, Carukia barnesi

Carukia barnesi

Carukia barnesi is a small and extremely venomous jellyfish found near Australia. Stings can result in Irukandji syndrome, and thus this species is commonly known as Irukandji jellyfish, although this name does not distinguish it from other Irukandji jellyfish such as Malo kingi.A mature C....

and Malo kingi

Malo kingi

Malo kingi is an Irukandji jellyfish named after victim Robert King, a tourist from the United States who died from its sting. It was first described to science in 2007, and is one of two species in genus Malo. It has some of the world's most potent venom, even though it is no bigger than a human...

are among the most venomous creatures in the world. Stings from these and a few other species in the class are extremely painful and sometimes fatal to humans.
"Box jellyfish" or "sea wasp" is also a common name

Common name

A common name of a taxon or organism is a name in general use within a community; it is often contrasted with the scientific name for the same organism...

 for the notoriously dangerous Chironex fleckeri

Chironex fleckeri

Chironex fleckeri, commonly known as sea wasp, is a species of Box jellyfish found in coastal waters from northern Australia and New Guinea north to the Philippines and Vietnam. It has been described as "the most lethal jellyfish in the world", with at least 63 known deaths from 1884 to...

.