annulus

A ringlike figure, part, structure, or marking, such as a growth ring on the scale of a fish.

A ring or group of specialized cells around the sporangia of many ferns. By changing shape in response to variations in humidity, it breaks open the sporangium and then releases the spores with a whipping motion.

The ringlike remains of a membrane (called a veil ), found around the stipes of certain basidiomycete mushrooms. The presence or absence of an annulus is often used to identify the species of an individual mushroom.

The figure bounded by and containing the area between two concentric circles.