36 MORPHOLOGICAL TYPES 



Absence of dermal skeleton (fin-rays or teeth) ; 

 Absence of special stomach ; 

 Absence of special genital ducts ; 

 Absence of medullated nerves. 



Specialised characters of Petromyzon : 



Rasping tongue ; 

 Sucking mouth ; 

 Horny teeth ; 

 Sac-like gill-pouches ; 

 Separate branchial duct ; 

 Large hypophysial sac ; 



Single median dorsal pore for nasal organs and hypo- 

 physial sac. 



Literature. 



Goodrich, E. S. Vertebrata Craniata, Cyclostomes and Fishes. Black, 

 London, 1909. 



Parker, T. J. A Course of Instruction in Zootomy (Vertebrata). Mac- 

 millan, London, 1884. 



