25 

 Aquatic Vertebrates 



Cyclostomes and Fishes 



The three most primitive classes of the vertebrates contain animals 

 that are aquatic in nature — that is, they spend practically their entire 

 lives in the water and, as a rule, can live only a short time out of the 

 water. In this chapter we will survey some representative members 

 of these classes. 



Round-mouthed Eels — Class Cyclostomata 



It may seem peculiar that an animal would have a mouth which 

 it cannot close, yet this is exactly what is found in the cyclostomes — 

 the mouths are round and permanently open, for there are no jaws to 

 close them. The marine lamprey eel is a well-known representative of 

 this class. It is a long slender eel-like animal, but it should not be 

 mistaken for the true eel which is a bony fish. The lampreys are corn- 



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Fig. 25.1. A lamprey eel. 



mon along the Atlantic Coast and may also be found in fresh-water 

 rivers and streams where they go to reproduce. Many have gotten into 

 some of our larger lakes, especially the Great Lakes where they now 

 remain as permanent inhabitants. The lamprey has an unusual method 

 of feeding which is very efficient in spite of its lack of jaws. Teeth are 

 present in circles around the sucking mouth and help the animal to 

 hold onto its prey. Teeth are also found on the tip of the firm tongue. 

 When a hungry lamprey attaches itself to the body of a large fish by 

 suction, the tongue is moved back and forth like a saw by very powerful 

 muscles until a hole is cut in the body of the fish. Then the blood 

 and small particles of flesh are sucked into the digestive system of the 



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