Final answer:
Gills in fishes are composed of many thin plates known as gill filaments, which are important for the respiration process. These filaments contain gill lamellae that increase the surface area for gas exchange. The operculum is a muscular flap that covers and aids in the function of gills.
Step-by-step explanation:
Each gill in fishes is made up of many thin plates called gill filaments. These filaments play a crucial role in the process of respiration, where water is drawn over them, allowing oxygen to be absorbed from the water and into the bloodstream of the fish. Within these filaments, there are lots of even thinner structures called gill lamellae which increase the surface area for gas exchange.
Blood vessels run along the filaments and lamellae, carrying deoxygenated blood to the gills where it gets oxygenated. The protective muscular flap called the operculum covers the gills and aids in the pumping mechanism necessary for water to flow over the gills.
All bony fishes use gills for respiration, with the presence of a swim bladder for buoyancy control. In contrast, animals such as chitons from class Polyplacophora possess multiple gills within their mantle cavity for breathing. On a broader note, while gills are primarily for respiration in aquatic animals, structures like the swim bladder evolved to help in maintaining buoyancy.