PETROMYZON 35 



series of somites is therefore complete, and no segment 

 has lost its somite. The myotomes are W-shaped, which 

 condition can easily be derived from the V-shaped myotomes 

 of Amphioxus by the turning forwards of their upper and lower 

 ends. They are not subdivided into dorsal and ventral 

 portions as in higher forms. 



The characters of Petromyzon and Cyclostomes can be 

 analysed under three headings : those which show an advance 

 from the condition of Amphioxus, those which are primitive 

 when compared with higher forms, and those which are 

 secondary and specialised. 



Characters shown by Petromyzon, absent in Amphioxus and 

 typical of Craniata : 



Formation of a distinct head, brain, and skull ; 



Formation of a distinct heart ; 



Formation of pro- and mesonephric kidneys ; 



Epidermis several cells in thickness ; 



Dorsal nerves with ganglia ; 



Rudimentary vertebral column ; 



Myotomes W-shaped ; 



Rudimentary sympathetic nervous system. 



Characters shown by Petromyzon {and Cyclostomes) which 

 are primitive when compared with higher forms : 



Endostyle, ciliated groove and velum of the Ammocoete 



larva ; 

 Emergence of glossopharyngeal behind the cranium ; 

 Dorsal and ventral nerves separate and unconnected in 



each segment ; 

 Fourth segment forming a complete myotome : no 



myotomes lost ; 

 Notochord unconstricted by vertebral column ; 

 Kidney-tubules segmental in Myxine ; 

 Myotomes not divided into dorsal and ventral portions ; 

 Persistence (although slight) of the pronephros ; 

 Absence of biting jaws ; 

 Absence of paired fins ; 



