1. A thin wispy cloud: Cirrus clouds occur as narrow bands of tiny ice particles, that form at the highest and coldest point of the cloud region.
2. A slender tentacle with sensory or locomotive function, or a part resembling one: An arm of a barnacle can be of a cirrus form.
3. A filament or slender appendage serving as a foot, tentacle,barbel, etc.: Jill read about cirrus, which was a fused group of cilia that functioned like a limb on some protozoans.

A cirrus is a structure formed from a cluster of tuft of fused cilia, constituting one of the sensory or locomotor organs of certain ciliate protozoa.
4. The male copulatory organ of various invertebrates: The flatworm , for example, is described as having a cirrus.
5. A slender flexible animal appendage: ; Cirri are found on barnacles or crinoids,or on many insects and are often tactile.