UPvINOGENITAL ORGANS 



349 



1>. SPECIAL PAKT. 

 URINARY ORGANS. 



In Amphioxus a series (90 or more) of independent segmental 

 tubules are present on either side in the reduced section of the 

 ccelome situated on the dorsal side of the pharynx (" dorsal 



B 



FIG. 277. DIAGRAMMATIC TRANSVERSE SECTIONS THROUGH A, AMPHIOXUS, IN 

 THE BRANCHIAL REGION, AND B, AN ELASMOBRANCH EMBRYO, BASED ON 

 BOVERI'S FIGURES. 



In A, the section passes through a branchial cleft on the right side, and shows a 

 transverse section of the anterior limb of a nephridium (X) ; on the left, a 

 nephridium (X) is indicated showing its communication with the ccelome (B) 

 and with the atrial chamber (C). A, genital section of ccelome (an ovary is 

 indicated on the right side) ; 7), section of the ccelome which extends down 

 the branchial bars ; F, ventral aorta. 



la B, the section represents the pronephric region on the left, and the meso- 

 nephric region on the right. A, rudiment of a mesonephric tubule, the 

 blind end of which subsequently comes to open into the pro- (or meso-) 

 nephric duct (C) as indicated by the dotted lines on the right. B, nephro- 

 stome ; D, ccelome : F, subintestinal vein. 



In both figures, E, lumen of gut ; G, aorta ; H, portion of commissural Vessel 

 which comes into relation with the excretory system. 



coelomic canals"). Each of these tubules comes into close relation 

 with a branchial blood-vessel, possesses a varied number of lateral 

 branches, and opens on the one hand into the ccelome by several 

 ciliated funnels or nephrostomes ; and on the other by a single 

 aperture into the atrial or peribranchial chamber (p. 275), which 



