The renal or urinary system is one of the oldest and most important systems of organs in the differentiated animal body, as I have pointed out on several previous occasions (cf. Chapter XVII). We find it not only in the higher stems, but also very generally distributed in the earlier group of the Vermalia. Here we meet it in the lowest worms, the Rotatoria (Gastrotricha, Fig. 242), and in the instructive stem of the Platodes. It consists of a pair of simple or branching canals, which are lined with one layer of cells, absorb unusable juices from the tissue, and eject them by an outlet in the outer skin (Fig. 240 nm). Not only the free-living Turbellaria, but also the parasitic Suctoria, and even the still more degenerate tapeworms, which have lost their alimentary canal in consequence of their parasitic life, are equipped with these renal canals or nephridia. In the first embryonic structure they are merely a pair of simple cutaneous glands, or depressions in the ectoderm. They are generally described as excretory organs in the worms, but formerly often as “water vessels.” They may be conceived as largely-developed tubular cutaneous glands, formed by invagination of the cutaneous layer. According to another view, they owe their origin to a later rupture of the body-cavity outwards. In most of the Vermalia each nephridium has an inner opening (with cilia) into the body-cavity and an outer one on the epidermis.
Fig. 386—Rudimentary primitive kidneys of a dog-embryo. The hind end of the embryonic body is seen from the ventral side and covered with the visceral layer of the yelk-sac, which is torn away and folded down in front in order to show the nephroducts with the primitive urinary canals (a). b primitive vertebræ, c spinal cord, d entrance into the pelvic-gut cavity. (From Bischoff.)
Fig. 387—Primitive kidneys of a human embryo. u the urinary canals of the primitive kidneys, w Wolffian duct, w′ uppermost end of the same (Morgagni’s hydatid), m Mullerian duct. m′ uppermost end of same (Fallopian hydatid), g gonad (sexual gland). (From Kobelt.)
In these lowest, unsegmented worms, and in the unsegmented Molluscs, there is only one pair of renal canals. They are more numerous in the higher Articulates. In the Annelids, the body of which is composed of a large number of joints, there is a pair of these pronephridia in each segment (hence they are called segmental canals or organs). Even here they are still simple tubes; on account of their coiled or looped form they are often called “looped canals.” In most of the Annelids, and many of the Vermalia, we can distinguish three sections in the nephridium—an outer muscular duct, a glandular middle part, and an inner part that opens by a ciliated funnel into the body-cavity. This opening is furnished with whirling cilia, and can, therefore, take up the juices to be excreted directly from the body-cavity and convey them from the body. But in these worms the sexual cells, which develop in very primitive form on the inner surface of the body-cavity, also fall into it when mature, and are sucked up by the funnel-shaped inner ciliated openings of the renal canals, and ejected with the urine. Thus the urine-forming looped canals, or pronephridia, serve as oviducts in the female Annelids and as spermaducts in the male.
The renal system of the Vertebrates is similar to, yet materially different from, these segmental canals of the Annelids. The peculiar development of it and its relations to the sexual organs are among the most difficult problems in the morphology of our stem. If we examine briefly the vertebrate renal system from the phylogenetic point of view, as confirmed by recent discoveries, we may distinguish three forms of it: (1) Fore-kidneys or head-kidneys (pronephros); (2) primitive or middle kidneys (mesonephros); (3) permanent kidneys (metanephros). These three systems of kidneys are not fundamentally and completely distinct, as earlier students (such as Semper) wrongly supposed; they represent three different generations of one and the same excretory apparatus; they correspond to three phylogenetic stages, and succeed each other in the stem-history of the Vertebrates in such wise that each younger and more advanced generation develops farther behind in the body, and replaces the older and less advanced generation that preceded it in time and space. The fore kidneys, first accurately described by Wilhelm Müller in 1875 in the Cyclostomes and Ichthyoda, form the sole excretory organ of the Acrania (Amphioxus); they continue in the Cyclostomes and some of the fishes, but are found only in slight traces and for a time in the embryos of the six other classes of Vertebrates. The primitive kidneys are first found in the Cyclostomes, behind the fore kidneys; they have been transmitted from the Selachii to all the Gnathostomes. In the Anamnia they act permanently as urinary glands; in the Amniotes their anterior part (“germinal kidneys”) changes into organs of the sexual apparatus, while the third generation develops from the end of their posterior part (“urinal kidneys”)—the characteristic after or permanent kidneys of the three higher classes of Vertebrates. The order in which the three renal systems succeed each other in the embryo of man and the higher Vertebrates corresponds to their phylogenetic succession in the history of our stem, and, consequently, in the natural classification of the Vertebrates.