633 



CHAPTER VIII. 



URINARY SYSTEM OF HJSMATOCRYA. 



§ 95. Kkhieys of Fishes. — In all Vertebi'ates an excretory 

 organ is very early developed in the form of a tvibe, extending 

 from each side of the cloaca forward, along the dorsal region of 

 the abdomen, close to the spine, where it communicates with a 

 number of slender blind tubes jdaced at right angles to it ; the 

 longitudinal trunk- tube serving as the excretory duct of the 

 shorter transverse secerning ca3ca. These glands are transitory in 

 the air-breathing Vertebrates and are called, from their discoverer, 

 ' corpora WolfKana ; ' they are persistent in fishes ' and are 

 called ' kidneys,' fig. 352, n : in both they are renal organs and 

 secrete urine. 



A slightly oj^aque, slender, elongated glandular body, in the 

 situation marked /* in fig. 169, may represent the renal organ in 

 Brancliiostoma. The structure of this organ is more obvious in 

 the Myxinoids : it is double : each long duct, fig. .353, i, a, as it 

 extends from the cloaca through the abdominal cavity, sends off, 

 at regular but distant intervals from its outer side, a short wide 

 tube, ib. h, which communicates by a narrow opening with a blind 

 sac, ib. d. At the bottom of this sac or caicum there is a small 

 vaso-ganglion, fig. 353, 2, d, free on all sides save that by which 

 the vessels, ib. a, enter, and ib. J,^ quit it : there are no urini- 

 ferous tubes in this vaso-ganglion : the contents of the ca3Ciim 

 must react through its thin parietes and those of the capillaries 

 with which it is in contact upon the blood in those capillaries, and 

 extract therefrom the azotised uric princiiile. Analogous vascu- 

 lar bodies, formed chiefly by convoluted tufts of arterial capilla- 

 ries, are present in the Wolffian bodies of Mammals, and in the 

 persistent renal organs of all Vertebrates. They are called, after 

 their discoverer, ' Malpighian corpuscles,' and the uriniferous 

 tubes take their rise by a sacciform blind beginning applied over 

 the vascular tuft or ganglion.^ 



The combined secerning ca3ca and vaso-ganglia form in tlie 



' cxxx. ii. p. 314. ^ SXT. p. 13. = cxxxvii. 



