148 CHORDATA. 



Class I. Cyclostomi (Marsipobranchii). 



Eel-like forms without paired fins, with a rounded suctorial mouth 

 not supported by jaws, an unconstricted notochord and cartilaginous 

 skeleton, and six to seven pairs of branchial pouches. Nostril unpaired. 



Order 1. Hyperotreti. The nasal passage communicates with the mouth- 

 cavity. Myxine glutinosa, hag. 



Order 2. Hyperoartia. Nasal passage ends blindly. Petromyzon. 

 Larval form Ammocastes. P. Planeri ; P. fluviatilis, lampern ; P. 

 marinus, lamprey. 



