In biology, a cirrus (/ˈsɪrəs/ SIRR-əs, pl.: cirri, /ˈsɪraɪ/ SIRR-eye, from the Latin cirrus meaning a curl-like tuft or fringe) is a long, thin structure in an animal similar to a tentacle 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 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.
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