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In page Polyp (zoology):

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We can distinguish therefore in the body of a polyp the column, circular or oval in section, forming the trunk, resting on a base or foot and surmounted by the crown of tentacles, which enclose an area termed the peristome, in the centre of which again is the mouth. Generally, there is no other opening to the body except the mouth, but in some cases excretory pores are known to occur in the foot, and pores may occur at the tips of the tentacles. A polyp is an animal of very simple structure,[1] a living fossil that has not changed significantly for about half a billion years (per generally accepted dating of Cambrian sedimentary rock).[citation needed]