266 



ZOOLOGY 



SECT. 



CyAo 



surface of the tympanic membrane, its handle united to the 

 stapes (stp.), which is fixed in the membrane of the fenestra 

 ovalis (fen. ov.). Sonorous vibrations striking the tympanic mem- 

 brane are communicated by the columella and stapes to the 

 fenestra ovalis, thence to the perilymph, and thence to the 

 membranous labyrinth. The connection of the Eustachian tube 

 with the mouth obviates undue compression of the air in the 

 tympanic cavity. There seems little doubt that the tympano- 

 Eustachian passage is homologous with the first or hyomandibular 

 gill-cleft, although, in the Frog, it is formed independently of the 



clefts and never opens on the 

 exterior. 



Urinogenital Organs. The 

 kidneys (Figs. 883 and 884, N.) t 

 are flat, somewhat oval bodies of 

 a dark red colour, lying in the 

 posterior region of the coelome. 

 On the ventral face of each is an 

 elongated, yellow adrenal, and 

 irregularly scattered nephrostomes 

 occur in considerable numbers on 

 the same surface. They do not, 

 however, communicate with the 

 urinary tubules, but with the renal 

 veins, and serve to propel the 

 lymph from the coelome to the 

 venous system. The ureters (Ur.) 

 pass backwards from the outer 

 borders of the kidneys and open 

 into the dorsal wall of the cloaca 

 (CL). The kidney is developed 

 from the mesonephros of the 

 embryo, the ureter from the 

 mesonephric duct. In the larva a 

 large pronephros is present and is, 

 for a time, the functional kidney. 



Opening into the cloaca on 

 its ventral side is an organ 



(Fig. 876, bl.) mentioned in the general account of the Craniata 

 (p. 113), but here actually met with for the first time. It is a 

 bilobed, thin-walled, and very delicate sac into which the urine 

 passes by gravitation from the cloaca when the anus is closed. 

 The sac is a urinary bladder, but as it is quite different morpho- 

 logically from the organ of the same name in Fishes, which is a 

 dilatation of the ureter, it is distinguished as the allantoic bladder. 

 The testes (HO.) are white ovoid bodies lying immediately ventral 

 to the anterior ends of the kidneys, to which they are attached by 

 folds of peritoneum. From the inner edge of each pass a number 



FIG. 883. Rana esculenta. Urino- 

 genital organs -of the nlale. Ao. dorsal 

 aorta ; Cl. cloaca ; Cr. post-caval vein ; 

 FK, fat bodies ; HO. testes ; N, kidneys ; 

 S, apertures of ureters into cloaca ; Ur. 

 ureters. (From Wiedersheim's Com- 

 parative Anatomy.) 



