Gills are typically comprised of a gill arch, which contain veins and arteries that supply blood flow to the attached gill filaments. The arches are a rigid stucture which provide support and protection for the attached lamellae. There are usually two types of filaments, which are attached, primary and secondary.

Primary lamellae (or gill filaments) extend perpen

The total number of lamellae constitutes the total surface area of the gills available for gas transfer. The number of lamellae per animal is correlated with their size and activity, the larger and more active the animal the more lamellae it will have. (Evans 1998)

Gills provide a one-way flow for oxygen to perfuse over them. This one way flow increases their efficiency since there is not much mixing of oxygenated and deoxygenated water directly over the gills and there is no "dead air space" such as the trachea in which oxygenated and deoxygenated water can get mixed.

Secondary lamellae extend vertically from the primary lamellae (or filaments) and are placed closely together forming small channels for water to flow through. Each secondary lamella is made up of two sheets of epithelial cells with pillar cells that hold them apart. These pillar cells form

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