In the lowest adult form of the vertebrates, amphioxus, the gill-system is used largely as a sieve for procuring food, partly also, perhaps, for respiration. In the sharks, bony fishes, and lower amphibians, water is taken in through the mouth, and passes through the gill-slits to the exterior. As it goes through the slits it passes over the gills, that stand like fringes on the sides of the slits. The blood that passes in large quantities through the gills is aerated in this way. In the embryos of the higher vertebrates the gill-slits may appear even before the mouth has opened, but in no case is there a passage of water through the gill-slits, nor is the blood aerated in the gill-region, although it passes through this part on its way from the heart to the dorsal side of the digestive tract. It is quite certain that the gill-system of the embryo performs no respiratory function.[9]
This statement is not intended to prejudice the question as to whether the presence of the gill-slits and arches may be essential to the formation of other organs.
In the higher amphibians, the frogs for example, we find an interesting transition. The young embryo, when it emerges from the egg-membranes, bears three pairs of external gills that project from the gill-arches into the surrounding water. Later these are absorbed, and a new system of internal gills, like those of fishes, develops on the gill-arches. These are used throughout the tadpole stage for respiratory purposes. When the tadpole is about to leave the water to become a frog, the internal gills are also absorbed and the gill-clefts close. Lungs then develop which become the permanent organs of respiration.
There are two points to be noticed in this connection. First, the external gills, which are the first to develop, do not seem to correspond to any permanent adult stage of a lower group. Second, the transition from the tadpole to the frog can only be used by way of analogy of what is supposed to have taken place ancestrally in the reptiles, birds, and mammals, since no one will maintain that the frogs represent a group transitional between the amphibians and the higher forms. However, since the salamanders also have gills and gill-slits in the young stages, and lose them when they leave the water to become adult land forms, this group will better serve to illustrate how the gill-system has been lost in the higher forms. Not that in this case either, need we suppose that the forms living to-day represent ancestral, transitional forms, but only that they indicate how such a remarkable change from a gill-breathing form, living in the water, might become transformed into a lung-breathing land form. Such a change is supposed to have taken place when the ancestors of the reptiles and the mammals left the water to take up their abode on the land.