424 MANUAL OF ZOOLOGY SECT. 



space, the tympanic cavity, which communicates with the 

 mouth by the short Eustachian tube already noticed (Fig. 

 235, eus. /.), so that a probe thrust through the tympanic 

 membrane from outside passes directly into the mouth. In 

 the roof of the tympanic cavity lies a slender rod of bone 

 and cartilage, the columella its head, or txtra-columtlla, 

 attached to the inner surface of the tympanic membrane, its 

 handle united to the stapes (Fig. 232, stp\ a nodule of 

 cartilage which is fixed in the membrane of the fenestra 

 ovalis, an aperture in the outer wall of the auditory capsule 

 (? 35 ^)- 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. 



The kidneys (Figs. 235, 237 and 238, Kd., and Figs. 

 240 and 241, N.\ 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 kidney, and open into the dorsal 

 wall of the cloaca (Cl.). 



Opening into the cloaca on its ventral side is a bilobed, 

 thin-walled, and very delicate sac, the urinary bladder (Fig. 

 235, //.), into which the urine passes by gravitation from 

 the cloaca when the anus is closed. 



The testes (Fig. 240, Ho.) are white ovoid bodies lying 

 immediately ventral to the anterior ends of the kidneys, to 



