STRUCTURE OF THE VERTEBRATES 



31 



Petromyzontea, or lampreys, include Petromyzon marinus, 

 the form ordinarily used in laboratories for dissection. The 

 lampreys have a complete cartilaginous branchial basket and 

 seven gill pouches. During development a longitudinal constric- 

 tion separates the dorsal gullet from the ventral respiratory 



,Naso-pituitary 

 opening 



Petromyzon 



Ectodermal 

 teeth 



Anus 



Naso-pituitary groove 



Nerve cord 



Mouth 



Oral Hood 

 Petromyzon 



Heart' 



Mating Brook Lampreys 



Bdellostoma (Myxinoid Cyclostome) 



Fig. 7. Cyclostomes. Bdellostoma belongs to the more primitive order, 



the Myxinoidea. The section of the lamprey larva (Petromyzontia) shows 



the pinching off of the endostyle to form the thyroid. 



