In biology, a cirrus ( , : 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 main ...
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 (; : penises or penes) is a sex organ through which male and hermaphrodite animals expel semen during copulation (zoology), copulation, and through which male placental mammals and marsupials also Urination, urinate.
The term ''pen ...
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 stars or
barnacle
Barnacles are arthropods of the subclass (taxonomy), subclass Cirripedia in the subphylum Crustacean, Crustacea. They are related to crabs and lobsters, with similar Nauplius (larva), nauplius larvae. Barnacles are exclusively marine invertebra ...
s, a cirrus is a long slender gripping or feeding appendage.
In
sea lilies
Crinoids are marine invertebrates that make up the Class (biology), class Crinoidea. Crinoids that remain attached to the sea floor by a stalk in their adult form are commonly called sea lilies, while the unstalked forms, called feather stars or ...
, 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
A nautilus (; ) is any of the various species within the cephalopod family Nautilidae. This is the sole extant family of the superfamily Nautilaceae and the suborder Nautilina.
It comprises nine living species in two genera, the type genus, ty ...
, 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