What is the hard plate that opens at posterior end to cover and project gills of fish?

The hard plate that opens at the posterior end to cover and project the gills of fish is called the operculum.

Here's a breakdown:

* Operculum: A bony flap that covers and protects the gills in bony fish.

* Posterior end: The back or tail end of the fish.

* Project gills: The operculum helps to move water over the gills for respiration. When the fish opens its mouth, water flows in, and then the operculum closes, forcing the water over the gills and out the opening.

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