XX] MAIN DIVISIONS 455 



the walls of gill-sacs, and there is no common gill-cover to protect 

 the external openings of the gills ; the anterior external margin of 

 each gill-sac being produced into a slight flap which can close the 

 opening when it is pressed down. The skin is well covered with 

 placoid scales, and the teeth are not fused together. In the Holo- 

 cephali on the contrary, the upper half of the first visceral arch 

 which in Elasmobranchii forms, the upper jaw is indistinguishably 

 fused with the cranium. There is a common gill-cover, the opercu- 

 lum, developed as a flap of skin arising from the hyoid arch which can 





FIG. 221. Section through the skin of an Elasmobranch showing formation 



of a dermal spine. Highly magnified. 

 1. Horny layer of ectoderm. 2. Malpighian layer. 3. Columnar cells 



of ectoderm secreting 4. 4. Enamel. 5. Dentine (black). 6. Dentinal 



pulp. 7. Connective tissue. 



cover all the gill -slits, and the septa dividing the gill -slits from each 

 other are reduced in breadth, the gill-folds arising from the walls of 

 the gill-sacs project slightly beyond the outer edges of the septa. 

 The teeth are fused together to form great dentinal ridges with 

 specially hard parts called tritors and the skin is naked, placoid 

 scales being only developed on a peculiar tentacle which the male 

 has developed on his head and which he uses to seize the female 

 during copulation. 



Order I. Elasmobranchii. 



The Elasmobranchii includes the vast majority of fish classed as 

 Chondrichthyes, and we shall therefore describe the structure of a 

 typical Elasmobranch in order to give an idea of the organisation of 

 Chondrichthyd fish. In a typical Elasmobranch the skull is much 



