EXCRETORY SYSTEM. 



491 



with the abdominal cavity by two fine canals, and is an 

 anterior part of the ccelome. The blood contains, as usual, 

 red blood corpuscles and leucocytes. 



The dark red spleen lies in the curve of the stomach. 

 The red thyroid gland lies just in front of the anterior end 

 of the ventral aorta. The whitish thymus gland is a paired 

 structure lying dorsally above the gills. 



Excretory System. 



Excretory System {see pp. 458-462). — The elongated, dark 

 red kidneys lie posteriorly on each side of the vertebral 



Fig. 162. — Urinogenital organs 01 male Skate. (From a 

 specimen in Eclinburgli Museum of Science and Art.) 



7'., Testis ; .£/., epididymis ; 71. d., vas deferens : /i^., kidney ; v.s., 

 seminal vesicle; s.s., sperm sac; u.g.s., urinogenital sinus; CI., 

 cloaca. 



column. They are developed from the hind part of the 

 mesonephros. Several tubes from each kidney combine to 

 form a ureter. The two ureters of the male open into the 

 urinogenital sinus, whence the waste products pass out by 



