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A cirrus (plural cirri) is an articulated appendage projecting from an ossicle that forms part of the stalk of a crinoid. In sea lilies the stem forms a permanent attachment round which the cirri are arranged in whorls. The lowermost ones form a holdfast attaching the sea lily to the substrate.
During the development of comatulid crinoids (feather stars), the stalk is lost apart from its topmost segment which remains attached to the calyx (central disc). In these crinoids, the cirri are attached to this segment and project from the underside of the crown. There are usually many cirri and they are used to cling on to a hard surface or grip the substrate as the animal moves slowly about. In species that live on rocks they are robust and curved but in species that live on soft substrates they are long and slender.