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In biology, acirrus (/ˈsɪrəs/SIRR-əs,pl.:cirri,/ˈsɪraɪ/SIRR-eye, from the Latincirrus meaning acurl-like tuft or fringe) is a long, thin structure in an animal similar to atentacle but generally lacking the tentacle's strength, flexibility, thickness, and sensitivity.
In thesheep liver fluke, for example, thecirrus is the worm's muscularpenis and when not in use is retained within acirrus sac orpouch near the animal's head.[1] The same structure exists in the variousTaenia species of tapeworm. In theclam worms, however, the cirrus is the tentacular process or growth on each of the feet (parpodia), either thedorsal cirrus or theventral cirrus, and has nothing to do with reproduction.[1]: p. 431
Among thebristleworms, a cirrus is a tentacular growth near the head ornotopodium containing sense organs and may be either dorsal, ventral, or lamellar.[2] Among theribbonworms, thecaudal cirrus is a small thread-like growth at the posterior end of the worm.[3] Amongfeather stars orbarnacles, a cirrus is a long slender gripping or feeding appendage.[4]
Insea lilies, the cirri are the thin strands that line the animal's stalk. Among thetube blennies, a cirrus is a long growth extending from above the eye (asupraorbital cirrus) or extending below the neck-region (anuchal cirrus).[5] In anautilus, 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of a thin flexible cirrus and the corresponding hardened and protective cirrus sheath into which the cirri may be withdrawn.[6]
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