AMPHIBIA. 



405 



each lung in the two last-named Amphibians and in Menopoma to the dorsal 

 wall of the coelome. 



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 Miillerian 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 my- 

 omere, 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 anteriorlyjn 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 a 



