Cyclostome 



B. Shark 



perctdum 



C. Teleost 



Fig. 121. External Appearance of the Gill Region of Cyclostomes and 



Fish. In fish above the Elasmobranchii the gills are covered with a bony 



operculum. Compare Fig. 122. 



Operculum 



A. Hexanchus 

 (primitive shark) 



B. Teleost 



Fig. 



122. Horizontal Sections of the Pharynx. (A) Primitive Elasmo- 

 branch, showing the six gill slits, and (B) a typical Teleost with five 

 gill slits covered by an operculum. In the Elasmobranch the septa extend 



to the outside. 



