The Collaborative International Dictionary of English v.0.48:
Cirrus \Cir"rus\, n.; pl. Cirri. [L., lock, curl, ringlet.]
[Also written cirrhus.]
1. (Bot.) A tendril or clasper.
(a) A soft tactile appendage of the mantle of many
Mollusca, and of the parapodia of Annelida. Those near
the head of annelids are Tentacular cirri; those of
the last segment are caudal cirri.
(b) The jointed, leglike organs of Cirripedia. See
Annelida, and Polych[ae]ta.
Note: In some of the inferior animals the cirri aid in
locomotion; in others they are used in feeding; in the
Annelida they are mostly organs of touch. Some cirri
are branchial in function.
3. (Zool.) The external male organ of trematodes and some
other worms, and of certain Mollusca.
4. (Meteor.) See under Cloud.