CHORDATA. Balanoglossus has a very slightly developed excretory system. Two ciliated funnels pass to the exterior in the region of the collar, but no nephridia are known. The proboscis gland has been thought possibly to have an excretory function, but apparently it does not open to the exterior. In tunicates the excretory function has been ascribed to a mass of clear vesicles in the loop of the intestine. In them uric acid is pres ent. The so-called sub-neural gland may possibly also have secretory function. In Amphioxiis Boveri has discovered about ninety pairs of nephridia. They are short tubes, and open into the atrial cavity by a single opening for each tubule. The other end is in communication with the body-cavity by means of :t variable munlier of Ilay La ukester, Ilatcheek. and others have described other tubules in different regions of the adult or larva whose funetion is in doubt. Likewise on the floor of the atrial chamber there are grumps of cells which have been called renal CRANt.1.1%\. The nwivmry system of vertebrates is so intimately connected with the repro:Net k-e system that the two systems are frequently con sidered together under the title nrinoyeuital sys tem or oryalis.
The excretory organs of vertebrates are much more complicated than any we have so far consid ered. In its most highly developed form the ver tebrate excretory system consists of three sets of organs. The first set to arise, both in phyloge netic and ontogenetic development, arc known as the promcphos or 'head kidneys.' Usually they arise in a more anterior position than the other kidneys. In position as well as in several other respects they correspond to the segmental tubules of amphioxus. They are segmentally arranged like those of amphioxus and annnlates, but they are much fewer in numbers. They arise in the mesoderm of the anterior end of the •rlomic wall and each tubule opens into the ccelom by a ciliated funnel. They differ from the tubules of annulates and amphioxus, however, in that each tubule does not open directly to the exterior, but pours its secretion into a common duct—tlie seg mental duct—which in turn discharges into the cloaca. It has been suggested that the segmental tubules formerly poured their contents into a longitudinal groove situated on the exterior. By the sinking of the groove beneath the surface a tube was formed. The embryonic development of this duct gives little light as to its origin. In some cases it is formed by a growth backward from the pronephros. In others the mesoblast or even the hypoblast seems to take an active part in its formation. The head kidney is said to be the functional excretory organ in the fish Teiras fer and some of the other bony fishes. In Myxine
and Bdellostoma it persists throughout adult life, although evidently in a somewhat degenerate con dition; while in all the higher vertebrates ex cept turtles and crocodiles it is rudimentary even in embryonic life.
The head kidney then is present at some stage in the development of all vertebrates, although its appearance may be very fleeting in some forms, and it may be so rudimentary in others as never to be functional. \\lien the head kidney is func tional the mesonephros or 'middle kidney' appears later in larval development than in the cases in which the pronephros is only rudimentarily de veloped. The mesonephros consists of another series of tubules, developed from the mesoblast and usually in a position posterior to that of the head kidneys. These tubules likewise hecome con nected with the segmental duct. Below the Amninta the Wolflian body, is the permanent kidney. Upon the appearance of the mesonephros the nephridia of the head kidneys lose their con nection with the segmental duct. The tubules of the mesonephros also open into the body-cavity by ciliated funnels. The funnels are not always pres ent. They are in intimate relation with a glo meriting from the aorta. In selachians, Paul Meyer and Pdiekert have observed vessels which connect the dorsal aorta with the subintestinal veins. Such a blood-supply corresponds more nearly with that afforded by the segmental or gans of the amphioxus.
In many forms the segmental duet seems to divide, or at least two duets appear side by side. One retains its connections with the kidney tubules and is known as the Wolffian duct. The other, the Miillerian duct, opens into the body cavity on the one hand and as the oviduct serves to convey the eggs to the exterior. in the male the Wolffian duet becomes the genital duct. In the higher vertebrates still a third or permanent kidney arises posterior to the mesonephros, and is known as the metanephros. This is the `kid• my' of the higher vertebrates. The permanent kidney consists of coiled tubules, but a nephro stome is never present. Capillaries, forming glo meruli, are ab,o present. The duct, the ureter of the permanent kidneys of higher vertebrates (from reptiles to man), is formed from a diver tienbun which grows forward from the posterior end of the \Volition duet and connects with the posterior nephridia.
The urinary bladder of fishes is formed by a divert iculum of the ureter. In higher vertebrates the bladder, while it has the same function, has an entirely different origin from that of fishes, for it arises as a pouch on the ventral wall of the cloaca..