Polymorphism usually occurs, certain individuals having the form of avicularia or vibracula.
- Zooecia of Umbonula pavonella, showing a pair of minute avicularia on either side of the orifice of each zooecium.
The most singular of the external appendages found in the Polyzoa are the avicularia and vibracula of the Cheilostomata.
The avicularia and vibracula give valuable aid to the systematic study of the Cheilostomata.
Avicularia of this type occur in the common Flustra foliacea, in various species of Membranipora, and in particular in the Onychocellidae, a remarkable family common in the Cretaceous period and still existing.