Final answer:
The lid covering the retractable lophophore in bryozoans is called an operculum, serving as a protective mechanism for these aquatic invertebrates.
Step-by-step explanation:
If the lophophore on bryozoans is able to retract, it has a lid called an operculum. Bryozoans are aquatic invertebrate animals that feed by filtering particles out of the water using their tentacle-lined lophophore. The operculum serves as a protective feature, covering the lophophore when it is retracted to help protect the delicate feeding structures from predation or debris.