annuses

Annulus, in biological terminology, denotes a ring-like structure found in various organisms and contexts. The word derives from Latin annulus meaning “little ring.” Its standard plurals are annuli or annuluses; annuses is a nonstandard form occasionally encountered.

In mycology, the annulus is the ring around the stem of a mushroom, left by a partially torn or shed veil during development. It can be persistent or quickly worn away, and its position, texture, and visibility vary among species. Mycologists often use the presence or absence of an annulus, as well as its characteristics, as one of several features for identifying mushrooms in genera such as Amanita and Agaricus.

In botany, particularly in ferns, the annulus refers to a ring of specialized cells surrounding the sporangium. This ring changes shape as the sporangium dries, creating a mechanism that helps release spores. The exact structure and behavior of the annulus can differ among fern groups, influencing how and when spores are shed.

In human anatomy, the term appears in contexts such as the annulus fibrosus, the tough outer ring of the intervertebral disc that encases the nucleus pulposus. The integrity of this annulus is important for disc function, and rupture or degeneration can contribute to back pain and disc herniation. The word also appears in descriptions of ring-like ligaments or structures elsewhere in anatomy.

Overall, annulus is a versatile term used across disciplines to describe circular or ring-shaped components. The specific meaning depends on the biological context, from fungal stems to plant sporangia to spinal anatomy.