222 



CYCLOSTOMATA 



from the left side. The sinus venosus brings the impure blood to 

 the right side of the heart. Circulation is a thoroughly closed 



system. 



Cyclostomata. (Gr., a circle; a mouth.) — The Cyclostomes, 

 forms just below the fishes, include hag-fish and lampreys, both of 

 which somewhat resemble eels but differ in a number of essential 

 characteristics. (Figure 103.) 



Fig. 103. Cyclostomes. Upper figure, Pacific hagfish, Bdellostoma dombeyi. 

 X 3^. The light apertures along the sides are mucous canals, the dark ones are 

 branchial openings. Middle figure, Atlantic hagfish, Myxine glutinosa. X K- The 

 dots along the side are mucous pits; the left common branchial aperture is at *. Lower 

 figure, sea lamprey, Petromyzon marinus. X 3^. (After Dean.) 



Characteristics. — i. No jaws. 



2. No lateral appendages. 



3. No scales. 



4. No masticatory apparatus; rasping tongue present. 



5. No atrial cavity. 



6. Have a median unpaired nostril and the first distinct ap- 

 pearance of a head. 



7. Have round mouth closed by the tongue. 



8. Pocket-like gills. 



9. Persistent notochord. 



10. Vertebrae present, separated from notochord. 



11. The gonads discharge into the coelom. 



Myxinoids. — The hag-fishes produce slime and, when captured, 

 " turn water into glue." They are all marine. They attack dis- 

 abled fish and enter the gills or mouth. Their digestive apparatus 

 is so large that one meal takes a long time to digest. Blind, they 



