A pronephros connected with the anterior end of the segmental duct forms the excretory apparatus of the Tadpole. It atrophies after a time, and the segmental duct becomes split into the Wolffian duct of the meso-nephros, and the Müllerian duct. The mesonephros in the young Caecilia (Gymnophiona) consists of a segmental tube with a nephrostome and Mal-pighian body corresponding to each myomere. At a later period the posterior tubes become compound, while the anterior retain their primitive arrangement, and the mesonephros forms a narrow band of great length. In the Urodela the mesonephros is divisible into a narrower anterior sexual region and a broader posterior or excretory region. In the lower forms the distinction is not so marked as in the higher, in which the anterior section may exceptionally be cut off (Salamandra perspicillata). There appears to be in development more than one segmental tube to each myomere, the number increasing posteriorly, but while in the sexual region the tubes are single and open independently into the Wolffian duct, they are compound in the posterior region. There are a number of separate collecting tubes to the posterior region of the mesonephros in Urodela which open into the Wolffian duct as a rule only near its cloacal extremity.
The number of nephrostomes is great, and two may open in the posterior region into the same tube or a single nephrostome lead into a canal which divides. These abnormal relations become the rule in Anura where the nephrostomes are connected in the adult with the renal-portal capillaries (?). In the Anura the mesonephros is broad and a segmental arrangement of its tubes is not traceable. The Wolffian duct is dilated in the male, where it becomes free from the gland. This dilatation acts as a vesicula semi-nalis (see p. 78). The kidneys are covered by peritoneum only on their ventral surface, to which the nephrostomes are confined, in Anura and Urodela, on both dorsal and ventral in Gymnophiona. The Wolffian ducts open separately on the dorsal wall of the cloaca. An allantoid bladder, or ventral outgrowth of the cloaca, is invariably present. It has an anterior and posterior horn in some Gymnophiona, and is bifurcated anteriorly in all Anura and the Urodela with a few exceptions (some Salamanders and the lower forms).
The sexual organs, male and female, lie near the inner border of the kidneys, suspended by peritoneal folds: in Gymnophiona, however, to their outer or ventral sides. The testes in Gymnophiona consist of a series of more or less oval bodies one behind the other, traversed and connected by a common collecting tube. The tubuli seminiferi open into this tube, from which transverse canals, arising one between each testicular body, pass to a longitudinal canal lying near the inner border of the kidney. From this in turn a series of transverse canals arise which are connected each to aMalpighian body. In other Amphibia the testes are compact bodies. The Urodela have an internal collecting tube for the seminal tubes, which is wanting in Anura, where there is generally an intratesticular network of vessels. The collecting tube of Urodela is connected by a variable number of transverse canals to a longitudinal canal (supra), and this in its turn by other transverse vessels to the Malpighian bodies of the sexual region of the mesonephros. The longitudinal canal is sometimes absent (Spelerpes) and a direct connection then occurs between testis and kidney. The longitudinal canal is present in many Anura, but in Bufo only do the transverse canals become connected with Malpighian bodies.
For Rana, see p. 78. In Bombinator most of the transverse vessels end blindly, and the anterior are connected directly to the Wolffian duct. In Discoglossus a few vessels connect the testis to the same duct, but in Alytes obstetricans the extra-testicular network is connected to the Miillerian duct, which ends blindly anteriorly, and opens posteriorly into the Wolffian duct. The Miillerian duct always exists in the male. In Gymnophiona it may retain its abdominal opening; it is extremely glandular posteriorly, as in Bufo, and each duct opens into the cloaca separately. In Bufo the two ducts unite first and then open into the cloaca. It always loses in Urodela its cloacal connection; it sometimes preserves, sometimes loses, both its abdominal opening and its cavity. In Anura, except Bufo, it is reduced to a solid cellular cord. Some male Toads, e. g. Bufo cinereus, have a rudimentary ovary. The ovary in Gymnophiona is a long thin band; in Urodela a cylindrical sac, and in Anura a sac subdivided by complete partitions into a series of more or fewer compartments.