xm 



PHYLUM CHORDATA 



161 



the urinogenital sinus, some of the most anterior first uniting to 

 form a common tube. The sinus has a median aperture into the 

 general cavity of the cloaca situated on the summit of a prominent 

 urinogenital papilla. The oviducts (Miillerian ducts) of the female 

 are represented in the male by vestiges of their anterior portions 

 (m.d.). The entire kidney is sometimes regarded as a mesonephros, 

 but the posterior portion, developed entirely behind the portion 

 which, in the male, takes part in forming the epididymis, and having 



FIG. 836. Hemiscyllium. Right kidney 

 and urinary sinus of female, med. ur. sinus, 

 median urinary sinus ; neph. kidney; 

 ur. sinus, right urinary sinus. 



FIG. 837. Dog-fish, egg-case. (After 

 Dean.) 



its own ducts, is sometimes looked upon as foreshadowing the 

 metanephros of the higher Vertebrates. 



The ripe ovum, rupturing the wall of its follicle, escapes 

 into the abdominal cavity, whence it reaches the interior of one 

 of the oviducts ; there it is fertilised by sperms received from the 

 male in the act of copulation, and then becomes enclosed in a 

 chitinoid case or shell (Fig. 837) secreted by the shell-gland. 



2. DISTINCTIVE CHARACTERS AND CLASSIFICATION. 



The Elasmobranchii are Pisces in which the cartilaginous cranium 

 is never ossified by replacing-bones, and in which investing-bones 



VOL. II L 



