im 



ZOOLOGY 



SKCT. 



mesonephros, but the posterior portion, developed entirely behind 

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

 and having its own ducts, is sometimes looked upon as fore- 

 shadowing the metanephros of the higher Vertebrates. 



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

 escapes into the abdominal cavity, whence it reaches the interior 

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



med.ur. sin- 



Pig. 826.— Hemiscyllium. Right kidney 

 and urinary sinus of female, incd. ur. miuR, 

 median urinary sinus ; iie'ph. kidney ; 

 ur. sinus, right urinary sinus. 



Fio. 826.— Dog-fisb, egg-case. (After 

 Dean.) 



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

 in a chitinoid case or shell (Fig. 826) 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 are not developed in connection either with the cranium or 

 the pectoral arch. The skull is hyostylic, except in some of the 

 Protoselachii, in which it is amphistylic (p. 78). The dermal 

 fin-rays are " horny " ; at their bases the fins are supported by 

 cartilaginous pterygiophores which are never very numerous. The 



