6i8 



STRUCTURE OF VERTEBRATES 



teleosts there is in the male an anterior part which ceases to be 



excretory and becomes sexual in function, and helps to convey 



the sperms to the exterior. The Wolffian duct is also a sexual 



duct, but in some, such as the 



toad and dogfish, there is a more 



or less complete split of the ducts 



into sexual and excretory parts. 



This tendency is carried further 



in the amniotes. Here a new 



duct, the ureter, grows forward 



from the mesonephric duct near 



its lower end, and round the 



head of this new channel a mass 



of unsegmented nephrogenic 



tissue forms a complex series of 



branched tubules, which never 



open to the splanchnocoele. This 



is the metanephros, and the 



ureter is the metanephric duct. 



It is probably best not to use 



these terms except for structures 



in the amniotes, but it must be 



recognised that many anam- 



a,.b. 



pyc. 



Fig. 475. — A semi-diagrammatic view 

 of the contents of the abdominal 

 cavity of a male cod which has been 

 opened on the right-hand side. 



a.b., Air-bladder; an., anus; bl., bladder; d., 

 duodenum ; g., genital opening ; g.b., gall- 

 bladder ; int., ileum ; kd., kidney duct ; Ir., 

 liver; ces., oesophagus; py.c, pyloric caeca; 

 rm., rectum ; st., stomach ; t., testis ; ur., 

 urinary opening. 



Fig. 476. — The viscera of the Sand 

 Lizard {Lacerta agilis), in their 

 natural relations. — From 

 Weidersheim. 



Bl., Bladder ; Ci., inferior vena cava ; ED., 

 rectum ; GB., gallbladder ; H., heart ; 

 Lg., Lg'., lungs ; M., stomach ; MD., 

 small intestine ; Oe., oesophagus ; Pn., 

 pancreas ; Tr., trachea. 



niotes have a closely comparable arrangement which cannot 

 formally be separated. The terms vas deferens and ureter may 

 justifiably be used for separated portions of the Wolffian duct 

 which are respectively sexual and excretory in function. 



