Acanthemblemaria aspera, Adult Female (Roughhead Blenny)

In biology, a cirrus , plural ''cirri'', , (from the Latin ''cirrus'' meaning a ''curl-like tuft or fringe'') is a long, thin structure in an animal similar to a

tentacle In zoology, a tentacle is a flexible, mobile, and elongated organ present in some species of animals, most of them invertebrates. In animal anatomy, tentacles usually occur in one or more pairs. Anatomically, the tentacles of animals work ma ...

but generally lacking the tentacle's strength, flexibility, thickness, and sensitivity. In the sheep liver fluke, for example, the ''cirrus'' is the worm's muscular

penis A penis (plural ''penises'' or ''penes'' () is the primary sexual organ that male animals use to inseminate females (or hermaphrodites) during copulation. Such organs occur in many animals, both vertebrate and invertebrate, but males d ...

and when not in use is retained within a ''cirrus sac'' or ''pouch'' near the animal's head. The same structure exists in the various ''Taenia'' species of tapeworm. In the clam worms, however, the cirrus is the tentacular process or growth on each of the feet (''parpodia''), either the ''dorsal cirrus'' or the ''ventral cirrus'', and has nothing to do with reproduction. Among the bristleworms, a cirrus is a tentacular growth near the head or notopodium containing sense organs and may be either dorsal, ventral, or lamellar. Among the ribbonworms, the ''caudal cirrus'' is a small thread-like growth at the posterior end of the worm. Among

feather star Crinoids are marine animals that make up the class Crinoidea. Crinoids that are attached to the sea bottom by a stalk in their adult form are commonly called sea lilies, while the unstalked forms are called feather stars or comatulids, which ar ...

s or barnacles, a cirrus is a long slender gripping or feeding appendage. In sea lilies, the cirri are the thin strands that line the animal's stalk. Among the tube blennies, a cirrus is a long growth extending from above the eye (a ''supraorbital cirrus'') or extending below the neck-region (a ''nuchal cirrus''). In a

nautilus The nautilus (, ) is a pelagic marine mollusc of the cephalopod family Nautilidae. The nautilus is the sole extant family of the superfamily Nautilaceae and of its smaller but near equal suborder, Nautilina. It comprises six living species in ...

, each of the animal's tentacles is composed of a thin flexible cirrus and the corresponding hardened and protective cirrus sheath into which the cirri may be withdrawn.

References

{{Zoology Animal anatomy Annelid anatomy