Gills are highly vascularised extensions of gas exchange membranes. The simple type may be extension of the body surface as in sea-slugs, sea stars and many of slow moving aquatic animals. The complexity of the gills reflects the animal's requirement for oxygen. Animals with greater need or oxygen have elaborate gill structure. Before we discuss the structure of the gill let us consider the ways in which gills are ventilated because the environment in contact with respiratory surface must be continuously renewed to avoid depletion of oxygen or excessive accumulation of carbon dioxide.