CHAPTER XIII 



BACKBONED ANIMALS 



1. Enteropneusts 2. Tunicates 3. Lancelets 4. Round-mouths 

 or Cyclostomes 5. Fishes 6. Amphibians 7. Reptiles 8. 

 Birds 9. Mammals. 



ACCORDING to Aristotle, fishes and all higher animals 

 were " blood-containing," and thus distinguished from 

 the lower animals, which he regarded as " bloodless." He 

 was mistaken as to the absence of blood in lower animals, 

 for in most it is present, but the line which he drew 

 between higher and lower animals has been recognised 

 in all subsequent classifications. Fishes, amphibians, 

 reptiles, birds, and mammals differ markedly from 

 molluscs, insects, crustaceans, " worms," and yet simpler 

 animals. The former are backboned (Vertebrate), the 

 latter backboneless (Invertebrate). 



It is necessary to make the contrast more precise, (a) 



FIG. 82. DIAGRAM OF " IDEAL VERTEBRATE," SHOWING THE SEGMENTS 

 OF THE BODY, THE SPINAL CORD, THE NOTOCHORD, THE GlLL-CLEFTS, 

 THE VENTRAL HEART. 



(After Haeckel.) 



Many Invertebrates have a well-developed nerve-cord, 

 but this lies on the ventral surface of the body, and is 

 connected anteriorly, by a ring round the gullet, with 

 a dorsal brain in the head. In Vertebrates the whole 

 of the central nervous system lies along the dorsal surface 



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