CHAPTER XXI 



COLD-BLOODED VERTEBRATA 1 



The dogfish and the frog are examples of two of the 

 principal classes of backboned animals^ — the 

 c e iti ebrS d te8— Fishes, or Pisces, and the Amphibia. - The 

 warrn-biooded.members of these groups, unlike the Birds 

 (Aves) and Suckling Animals, or Mammalia, 

 which we shall study in later chapters, are cold-blooded. 2 

 There exist two further classes of vertebrates, the Cyclosto- 

 mata and the Reptilia, both of which are also cold-blooded. 

 The cold-blooded vertebrates, other than the two which 

 we have already studied at some length, are of interest 

 to us, not only in themselves, but also because of the 

 light which is thrown by their anatomy upon that of the 

 more elementary laboratory types, and indeed of Man 

 himself. We shall see that some of them link the frog to 

 mammals, others bridge the gap between the frog and the 

 dogfish and lead downwards from the latter to the lancelet, 

 and yet others exhibit, well developed, organs which are 

 lacking or insignificant in the dogfish and frog. 



The Cyclostomes are a small group, of low organisation, 



whose members are fish-like but differ from all 



'other Vertebrata in being, like Amphioxus, 



without any trace of jaws or paired limbs at any stage of 



their lives, and in having, in place of the usual pair of nasal 



organs, a single median passage, which leads into a sack 



1 It has been assumed in writing this chapter that it will usually be 

 studied after Chapter XXIII., but it is not necessary that this order 

 should be observed. 



2 That is, in them the temperature of the body varies with that of its 

 surroundings. See pp. 60 and 453. Invertebrate animals are also cold- 

 blooded. 



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