CHAPTER XXI. 



PHYLUM CHORDATA. 

 SUB-PHYLUM CRANIATA. 



CLASS CYCLOSTOMATA. 

 (Synonym^ MARSIPOBRANCHII.) 



THE hag (Myxine), the lamprey (Petromyzon), and a few 

 others like them, differ in so many ways from Fishes, that 

 they must be ranked in a distinct class. They represent an 

 archaic type, whose interest has been enhanced by the 

 discovery of Palaospondylus in the Old Red Sandstone. 



GENERAL CHARACTERS. 



Unlike all higher Vertebrates (Gnathostomatd], the 

 Cydostomata have round suctorial mouths, without distinctly 

 developed jaws. They are also without paired fins and 

 without scales. Their respiratory system consists of paired 

 gill-pouches, to which the term Marsipobranch refers. The 

 body is vermiform, the unpaired fins have no true fin-rays. 

 In the extant forms the skeleton is wholly cartilaginous, and 

 the notochord persists unconstricted. The nasal organ is 

 unpaired, there is no sympathetic nervous system, no conus 

 arteriosus, no distinct pancreas, no spleen, no genital ducts, 

 and the segmental duct persists as such. Their geographical 

 distribution is wide. 



FIRST TYPE. Myxine The Hag. 



The glutinous hag (Myxine glutinosa) is not uncommon 

 off the coasts of Britain and Scandinavia, the Atlantic 

 coast of America, etc. It lives in the mud at depths of 



