The name “cottonmouth” is from the whitish lining of its mouth. When alarmed, it opens its mouth widely, showing the cotton-white lining. The body is black with little or no pattern or dark brown with darker bands on the back. Belly is dark brown or black. Young cottonmouths are patterned something like a copperhead and usually have a yellowish-green tail. Like all venomous snakes in Missouri, cottonmouths have a hole between the nostril and the eye, and the pupils are vertical, like a cat’s.