Gills, Podia and Papulae

Abstract

Gills are to be found in only four invertebrate phyla: annelids, arthropods, echinoderms and molluscs. In some hemichordates and in protochordates gill slits are present, but no gills. The term ‘gill’ can be applied to any outgrowth of the body wall that is primarily concerned with increasing oxygen uptake from the surroundings, and includes in this context not only the serially repeated gills of annelids, arthropods and certain molluscs and the ctenidia of the higher molluscan classes, but also the respiratory podia of irregular echinoids and the papulae of asteroids. It has been suggested that it should be limited to those structures which take up more oxygen per unit area than the general body surface does. It could be asked at this point whether increase in surface area or increase in oxygen uptake per unit area came first, but since this may vary in different animals it would seem more logical to consider the primary function of an organ, rather than the course by which this function has been achieved. It does not matter from the point of view of an animal’s efficiency whether a small gill is developed which effectively takes up more oxygen per unit area or whether a larger gill is developed whose oxygen consumption per unit area is the same as that of the general body surface.