A cirrus (pronounced //, "SEER-əs", plural cirri, //, "SEER-eye"), from the Latin cirrus meaning a curl-like tuft or fringe, is a term used in biology and zoology for a number of long, thin structures in both vertebrate and invertebrate animals. In the sheep liver fluke, for example, the cirrus is the worm's muscular penis 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 Nereidae, however, the term cirrus refers to tentacular processes or growths on each of the feet (parpodia), either the dorsal cirrus or the ventral cirrus, and has nothing to do with reproduction.:p. 431 Among the ribbonworms, the caudal cirrus is a term for a small thread-like growth at the posterior end of the worm. 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 feather stars or barnacles, a cirrus is a long slender gripping or feeding appendage. In sea lillies, the cirri are the thin strands that line the animal's stalk. Among the tube blennies, a cirrus is a term for a long growth extending from above the eye (a supraorbital cirrus) or extending below the neck-region (a nuchal cirrus). In a nautilus, 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of a thin flexible cirrus and the corresponding hardened and protective cirrus sheath into which the cirri may be withdrawn.
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