XIII 



PHYLUM CHORDATA 



215 



Urinogenital Organs. The kidneys (Fig. 865, M. t and Fig. 

 869, J?) are of great size, extending the whole length of the dorsal 

 wall of the abdomen, above the air-bladder, and partly fused 

 together in the middle line. They are derived from the meso- 

 nephros of the embryo. Their anterior ends (Fig. 865, Jed, Fig. 

 869, K) are much dilated and consist in the adult of lymphatic 

 tissue, thus ceasing to discharge a renal function. The ureters 

 (mesonephric ducts, ur.) unite into a single tube, which is dilated 

 to form a urinary Madder (Fig. 865, 

 u. U., Fig. 869, v.\ and discharges into 

 the urinogenital sinus. 



The gonads are of great size in the 

 sexually mature fish. The testes (Fig. 



CL.S.C 



p.s.c 



Fir;. 868. Salmo fario. The right auditory organ, 

 from the inner side ; the otoliths are shown 

 separately below. a. s. c. anterior semicircular 

 canal ; aud, nv. auditory nerve ; h. s. c. horizontal 

 canal ; ot. 13, otoliths ; p. s. c. posterior canal ; 

 sac. sacculus ; ut. utriculus. 



FIG. 869. Salmo fario. The kidneys 

 andj adjacent parts. d, precaval 

 vein ; R (to the right) kidney; R (to 

 the left), degenerate anterior portion 

 of kidney ; rr, efferent renal vein ; 

 s. subclavian vein ; u, ur, ureter ; v, 

 bladder. (From Gegenbaur's Com- 

 parative Anatomy.) 



865, ts.~) are long, smooth, pinkish, paired organs, extending the 

 whole length of the abdominal cavity ; each is continued pos- 

 teriorly into a duct (v. df.) which opens into the urinogenital 

 sinus, and the homology of which with the ducts of the primitive 

 nephridial system is still uncertain. The ovaries are also of 

 the full length of the abdominal cavity and are much wider 

 than the testes : they are covered with peritoneum on their 

 inner or mesial faces only, and the numerous ova, which are about 

 4 mm. in diameter, are discharged when ripe from their outer 

 faces into the ccelome. There are no oviducts, but the anterior 

 wall of the urinogenital sinus is pierced by a pair of genital pores 



o 2 



