External gills are very simple respiratory structures: just outgrowth of skin which project into the water. They are, however,
• Highly branched and convoluted, exposing a large gaseous exchange surface to water; and
• Richly supplied with blood capillaries.
External gills are found in certain sea slugs and many fish and amphibian larvae. Water only circulates over the gills when these organisms move. Movement, on the other hand, is hampered by the highly branched gills. Thus, external gills are not particularly efficient in obtaining oxygen.
To function more efficiently, chambers were evolved to enclose gills and circulate water over them. Simple forms of such enclosed gills are seen in aquatic snails and crustaceans. In an aquatic snail, the gills are enclosed in a chamber, the mantle cavity, which opens to the outside. Water is pumped in and out of this cavity exposing the gills to a continuous stream of water. The oxygen uptakes is more efficient in this type of respiratory structures.
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