62 MORPHOLOGICAL TYPES 



The endostyle is not present as such, but it has been trans- 

 formed into the thyroid gland, situated beneath the floor of the 

 pharynx, as in the adult Petromyzon. In some dogfish, its 

 cells still bear cilia. 



The lining of the top of the gill-slits grows upwards to 

 form a number of paired glandular masses ; in close association 

 with the anterior cardinal veins they form the thymus glands. 

 Lying on the course of the sympathetic nerve-chains, there are 

 a number of bodies of the same origin and nature as the 

 sympathetic ganglion-cells. They are the supra-renal bodies 

 and originate from the nerve-tube. Posteriorly, between the 

 kidneys is an elongated structure, the inter-renal, which is 

 formed from the coelomic epithelium. It is interesting to 

 find these two sets of structures separate, for in higher forms 

 they combine to form the adrenals. 



Characters present in Scyllium, which are lacking in 

 Petromyzon : 



Biting jaws ; 



Paired fins ; 



Denticles (" placoid scales ") ; 



Dermal fin-rays (ceratotrichia) ; 



Definite stomach and pancreas ; 



Mixed spinal nerves (dorsal and ventral roots joined) ; 



Vertebral column constricting the notochord ; 



Dorsal ribs ; 



Myotomes separated into dorsal and ventral portions ; 



Miillerian and Wolffian ducts ; 



Seven segments included in the skull. 



Characters of Scyllium which are primitive when compared 

 with higher forms : 



Absence of bone ; 



Absence of swim-bladder ; 



Gill-slits opening separately to the outside ; 



Heart with single auricle and single ventricle ; 



Separate supra-renals and inter- renals. 



