Editing Cnidaria (section)

===Basic body forms===
[[File:Cnidiria medusa and polip svg hariadhi.svg|thumb|250px]] 
[[File:Actinodiscus macro.JPG|thumb|right|250px|Oral end of [[actinodiscus]] polyp]]
Most adult cnidarians appear as either free-swimming [[Medusa (biology)|medusa]]e or [[Sessility (zoology)|sessile]] [[polyp (zoology)|polyp]]s, and many [[hydrozoa]]ns species are known to alternate between the two forms.

Both are [[Symmetry (biology)#Radial symmetry|radially symmetrical]], like a wheel and a tube respectively. Since these animals have no heads, their ends are described as "oral" (nearest the mouth) and "aboral" (furthest from the mouth).

Most have fringes of tentacles equipped with [[cnidocyte]]s around their edges, and medusae generally have an inner ring of tentacles around the mouth. Some hydroids may consist of colonies of [[zooid]]s that serve different purposes, such as defence, reproduction and catching prey. The [[mesoglea]] of polyps is usually thin and often soft, but that of medusae is usually thick and springy, so that it returns to its original shape after muscles around the edge have contracted to squeeze water out, enabling medusae to swim by a sort of [[jet propulsion]].<ref name="Ruppert" />