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[[Dosya:Close-up of clone war of sea anemones.jpg|sol|thumbnail|Sea anemones, ''Anthopleura sola'' engaged in war for territory]]

The internal anatomy of anemones is simple. There is a gastrovascular cavity (which functions as a stomach) with a single opening to the outside which functions as both a mouth and an anus: waste and undigested matter is excreted through the mouth/anus, which can be described as an incomplete gut. A primitive nervous system, without centralization, coordinates the processes involved in maintaining homeostasis as well as biochemical and physical responses to various stimuli. Anemones range in size from less than 1¼&nbsp;[[centimetre|cm]] (½&nbsp;[[inch|in]]) to nearly 2&nbsp;[[metre|m]] (6&nbsp;[[foot (length)|ft]]) in diameter.<ref>http://sheppardsoftware.com/content/animals/animals/invertebrates/seaanemone.htm</ref> They can have a range of 10 tentacles to hundreds.

 

The muscles and nerves in anemones are much simpler than those of other animals. Cells in the outer layer (epidermis) and the inner layer ([[gastrodermis]]) have microfilaments grouped together into contractile fibers. These are not true muscles because they are not freely suspended in the body cavity as they are in more developed animals. Since the anemone lacks a skeleton, the contractile cells pull against the gastrovascular cavity, which acts as a hydrostatic skeleton. The stability for this hydrostatic skeleton is caused by the anemone shutting its mouth, which keeps the gastrovascular cavity at a constant volume, making it more rigid.