Gill rays are found within the gill filaments, and provide support for the gill. Lamellae are oriented parallel to the flow of water through the gills, to maximize efficiency of diffusion. In addition, the blood flow through the gills opposes the flow of water through the lamellae. Such flow is called countercurrent flow, and maximizes the efficiency of diffusion. The Agnathans have external and internal pores rather than gill slits, and water is drawn into the gill chambers through the mouth and then passed over the gills.
Sharks have gill slits rather than pores, and gill septa that help support gill filaments.
The exception occurs when the shark is feeding, when water moves into the pharynx through the spiracle. The operculum, a bony gill covering, protects the filaments, and expiration occurs through a single gill slit. B. External gills External gills develop from the skin ectoderm of the bronchial area, but are not directly related to the visceral skeleton or branchial chambers. They are found most often in larval or paedomorphic amphibians. Lungs are found among fishes found in warm or stagnant water, as well as in primitive fishes, and allow the fish to gulp air and undergo diffusion in an environment with relatively low dissolved oxygen levels.