EXCRETING ORGANS. 629 



tubuli uriniferi, the same diameter throughout their entire 

 course. The secretion of all the tubuli of each kidney is poured 

 directly into the long narrow ureter, which, without forming 

 a pelvic enlargement, and often without forming a urinary 

 bladder, opens into the cloacal termination of the intestine, 

 posterior to the rectal opening, and posterior to the genital 

 openings, whether male or female, as in the embryos of higher 

 vertebrata. Frequently, however, a urinary bladder is 

 developed, which is comparatively small in fishes, its orifice 

 then receives the terminations of the two ureters, and it 

 opens by a short wide passage at the back part of the cloaca. 

 The long narrow kidneys of many osseous fishes are approx- 

 imated and more .or less united on the median plain, without 

 any anastomosis of their internal tubuli. In the plagios- 

 tome fishes, the kidneys are smaller and shorter, as in che- 

 lonian and other reptiles, and the ureters enter, as usual, with 

 the vasa deferentia into a common short urethral passage, 

 without forming a urinary bladder. The kidneys of fishes 

 thus already present a very large secreting surface for the 

 distribution of capillary blood-vessels, which are always much 

 more minute than the tubuli on which they spread, as in 

 other secreting organs. The numerous arteries which supply 

 the renal lobes of fishes, come off directly from the trunk of 

 the descending aorta, or from the intercostal arteries which 

 are given off from its sides, and the renal veins mostly enter 

 the vena cava, as it passes forwards between the lobes of 

 these organs. The venous blood distributed through the 

 lobes of the kidneys, by the branches of the great superior 

 spinal vein, is received also by the vena cava, thus forming a 

 renal portal circulation. 



The kidneys in amphibia are less elongated and less lobu- 

 lated in form than in most fishes, and the urinary bladder 

 is of greater size and more constant in its occurrence. These 

 glands originate early in the embryo, before the genital 

 organs; they are developed more towards the dorsal and pelvic 

 portion of the trunk than the corpora Wolffiana, and in the 

 adult state they neither extend forwards to the cranium, nor 

 backwards to the posterior end of the abdominal cavity, as 

 they do in most fishes ; so that the ureters have here a longer 

 free course before reaching the back part of the cloaca, 

 where they open at the sides of the wide orifice of the large 

 urinary bladder. They are at first, as in fishes, narrow, flat, 



