SECTION THREE 



CHAPTER 



8 



THE LOWER FISHES— Lampreys, Sharks, Rays, 



and Ratfishes 



The vertebrates— fishes, amphibians, reptiles, birds, and mammals— compose the 

 dominant and most advanced of the chordates (Chapter 7). This is an extremely 

 diverse group, united by the possession of at least traces of a backbone composed 

 of separate vertebrae. They also possess a cranium which encloses and protects 

 the specialized anterior end of the central nerve chord, the brain. It is 

 through the specialization of the brain and the allied sense organs (eves, nose, 

 ears, etc.) that the advanced vertebrates are able to respond more delicately 

 to subtle modifications of their environment than any other animals. Thus 

 they have become the dominant living organisms on the earth and in the sea. 

 It is not at all clear where the vertebrates came from. Probably some of the 

 prochordates had common ancestors with some vertebrates, but this occurred 

 so long ago (at least 500 million years) that the story is not preserved in fossil 

 form for us to interpret today. 



THE JAWLESS VERTEBRATES: Glass Agnatha {"imthout jaws") 



This group, widespread in the temperate zones of both hemispheres, includes 

 descendants of a once prominent array of fishes called "ostracoderms" which lived 

 over 300 million years ago. The ancient forms were mud-eating, bottom-living 

 marine animals which looked more conventionally fishlike than their eellike 

 descendants, the lampreys and hagfishes of today. These have survived probably 

 because they took up a very specialized mode of life— parasitism. It is a matter 

 of definition whether to call the lampreys and hagfishes "parasites," "carnivores," 

 or "scavengers." Theoretically, the parasitic habit begins with a stage in which 

 an animal merely partakes of part of the living host's tissues but lives a 

 largely independent life. The mosquito is an example. Gradually, as evolution 

 progresses, the dependence on the host increases until the parasite is virtually 

 completely dependent on the host. Such is the case with the tapeworm. 

 Lampreys and hagfishes are a borderline case. They are specialized in feeding 



B2 



