Gastric or internal lungs are only found in the higher animals, to which we give the name of quadrupeds (or _tetrapoda_), the amphibia and amniotes, and their fishlike ancestors, the dipneusta. These internal lungs are sac-shaped folds of the fore-gut, formed originally from the swimming-bladder (_nectocystis_) of the fishes by change of function. This air-filled bladder, a sac-shaped appendage of the gullet, merely serves the purpose of a hydrostatic organ, by varying the specific weight, in the fishes. When the fish wishes to descend it contracts the bladder and becomes heavier; it rises to the top by inflating it again. The lungs were formed by the adaptation of the blood-vessels in the wall of the swimming-bladder to the interchange of gases. In the oldest living lung-fishes (_ceratodus_) it is still a simple sac (_monopneumones_=one-lunged); in the others the simple gullet-cavity divides early into a pair of sacs (_dipneumones_, two-lunged). The wind-pipe (_trachea_--not to be confused with the organ of the same name in the tracheata) is formed by the lengthening of their stalk and strengthening of it with cartilaginous rings. At the anterior end of the trachea we find already formed in the amphibia the larynx, the important organ of voice and speech. The function of removing unusable matter is not less important to the organism than breathing. Just as breathing gets rid of the poisonous carbonic acid, so the kidneys remove fluid and solid excreta in the shape of urine; these are partly acid (uric acid, hippuric acid, etc.), partly alkaline (urea, guanine, etc.). In most of the cœlomaria special organs for removing these would be superfluous, as this is accomplished (like breathing) by the stream of water that is constantly passing through the whole body. But with the platodes we begin to find important excretory organs in the nephridia, a pair of simple and ramified canals which lie on either side of the gut, and open outward. These primitive renal canals are transmitted by the platodes to the vermalia, and by these to the higher stems of the cœlomaria. In the latter they generally open by special funnels into the inner body-cavity, which serves as first receptacle for the urine. Their outer opening sometimes (primarily) goes through the outer skin at the back (excretory pores), sometimes (secondarily) to the rectum, and so out through the anus. The oldest articulates, the annelids, have a pair of nephridia in each segment of the body; each renal canal, or segmental canal, consists of three sections, an inner funnel which opens into the body-cavity, a middle glandular section, and an external bladder that ejects the urine by contraction. The disposition of the renal system in the internally articulated vertebrates is very similar to this; but now complicated structures begin to appear, a pair of compact kidneys (_renes_), which are made up of a number of branching nephridia. Three generations of kidneys succeed each other,