XXIX URINOGENITAL ORGANS 395 



canals. Two of these, the anterior and posterior canals, are 

 vertical in position, and are united with one another at their 

 adjacent ends ; at the other end each is dilated to form a 

 bulb-like swelling, the ampulla. The third or horizontal 

 canal opens at each end into the vestibule, and has an 

 ampulla at its anterior end. The vestibule gives off a tube, 

 the endolymphatic duct, which opens on the top of the head. 

 The whole apparatus contains a fluid, the endolymph, in 

 which is a gelatinous substance enclosing calcareous par- 

 ticles or otoliths. Patches of sensory epithelium are found 

 in the vestibule and in the ampullae, and to these the fibres 

 of the auditory nerve are distributed. There seems little 

 doubt that the membranous labyrinth has not only an 

 auditory, but also an equilibrating function — i.e., that the fish 

 is enabled by its means to maintain its equilibrium in the 

 water, as is also the case with the statocysts of Invertebrates. 



The excretory and the reproductive organs of the dogfish 

 are so closely associated as to be spoken of together as the 

 urinogenital organs. The sexes are distinct, and the males 

 are distinguished externally by having a pair of large grooved 

 rods, the claspers, connected with the inner borders of the 

 pelvic fins. They are used, like the peculiarly modified first 

 and second pairs of pleopods in the male crayfish (p. 323), 

 as copulatory organs. 



The kidneys (Fig. 103, kd) are long, fiat, lobulated bodies 

 lying one on each side of the backbone in the posterior 

 part of the abdominal cavity. From the ventral surface of 

 each spring numerous delicate ducts or ureters (ur), some 

 of which unite, opening into a small unpaired chamber, 

 called in the female the urinary sinus and in the male the 

 urinogenital sinus {u. g. s), which opens into the cloaca. 



In the embryo the kidneys appear in the form of separate 

 segmentally arranged tubes (Fig. 99, JVph) having the 



