xii PHYLUM CHORDATA 357 



of the membranous labyrinth, and affect the terminations of 

 the auditory nerve-fibres. In the Lizard the tympanic 

 membrane is nearly on a level with the skin of the head, 

 and its position is conspicuously indicated by a brown 

 patch situated behind the eye. In the Rabbit the tympanic 

 membrane is more deeply sunk, and a wide passage, the 

 passage of the outer ear, leads to it from the exterior. The 

 ear of the Rabbit also differs from that of the Lizard in the 

 presence of the prominent auricle or pinna of the ear to 

 which reference has been already made. 



The kidney 's, or organs of renal excretion, though they 

 differ in form in the three examples, are not widely different 

 in essential structure. Their function is the secretion of 

 urine, which consists of water containing various nitrogenous 

 waste matters in solution. Essentially the kidney is a mass 

 of tubules by whose agency the process of secretion is 

 carried on, the whole being richly supplied with blood- 

 vessels. Eventually the tubules open into a duct, the 

 ureter. In the Lizard and the Rabbit there is present a 

 median thin-walled sac, the urinary bladder, in which the 

 urine is stored, to be discharged at intervals. In the 

 Rabbit the ureters open into the bladder, and the latter 

 opens on the exterior by a median canal, the urethra. In 

 the Lizard the ureters and the bladder have independent 

 openings into the cloaca, and the bladder is filled only by 

 regurgitation from the latter chamber. 



The sexes are distinct in all three. There are two testes, 

 each with its duct or vas deferens. In the female there are 

 two ovaries (one in some Dog-fishes), which are solid bodies 

 in which the ova lie imbedded. In the Dog-fish, when 

 mature, the ova are of large size, containing a great quan- 

 tity of food-yolk. The ova of the Rabbit are extremely 

 small, while those of the Lizard are of a size intermediate 



