the actual gas exchange takes place most often at gills.
the actual gas exchange takes place most often at gills.
where does the actual gas exchange take place most often?
gills
some multicellular organisms such as flatworms (platyhelminthes) are relatively large but very thin, allowing their outer body surface to act as a gas exchange surface without the need for a specialised gas exchange organ. flatworms therefore lack gills or lungs, and also lack a circulatory system. other multicellular organisms such as sponges (porifera) have an inherently high surface area, because they are very porous and/or branched. sponges do not require a circulatory system or specialised gas exchange organs, because their feeding strategy involves one-way pumping of water through their porous bodies using flagellated collar cells. each cell of the sponge's body is therefore exposed to a constant flow of fresh oxygenated water. they can therefore rely on diffusion across their cell membranes to carry out the gas exchange needed for respiration.
gas exchange
the hygric hypothesis was first proposed in 1953, making it the earliest posed hypothesis for the evolution of discontinuous gas exchange. the hygric hypothesis proposes that the discontinuous release of co is an adaptation that allows terrestrial insects to limit respiratory water loss to the environment. this hypothesis is supported by studies that have demonstrated that respiratory water loss is substantially higher in insects forced to keep their spiracles open, than those of the same species who exhibit discontinuous gas exchange. in addition, laboratory selection experiments on "drosophila melanogaster" have shown that more variable gas exchange patterns can emerge in populations of insects artificially selected for tolerance to dry conditions. however, water loss during discontinuous gas exchange is only limited during the flutter phase if gas exchange during the flutter phase is convective (or assisted by muscular contraction). from a water conservation perspective, if ventilation during the flutter phase occurs entirely by simple diffusion, there is no benefit to having a flutter phase. this has led to the belief that some other factor may have contributed to the evolution of discontinuous gas exchange in insects.
discontinuous gas exchange