How does an operculum function in cheilostome bryozoans?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Eurystomella bilabiata, a moss animal from the Order Cheilostomata. — moss animal *Eurystomella bilabiata*, a moss animal from the Order Cheilostomata.

In cheilostome bryozoans, the operculum functions primarily as a protective, closable lid for the orifice—the opening through which the lophophore (tentacles) and tentacle sheath emerge.1

Beyond its role as a simple cover, the operculum has evolved into specialized structures in certain zooid types:

Avicularia: In these specialized zooids, the operculum is proportionally larger and equipped with strong adductor muscles, effectively functioning as a jaw.1
Vibracula: In these forms, the operculum is modified into a long, whiplike seta (hairlike projection).1

While the exact functions of these specialized opercula are not fully understood, they are believed to assist in keeping the colony surface clear of debris and epizoites.1