CHAPTER XVI 



BACKBONED ANIMALS 



I. Balanoglossus — 2. Tunicates — 3. The Lancelet — 4. Round- 

 Mouths or Cyclostomata — 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) 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 Verte- 

 brates the whole of the central nervous system lies along the dorsal 



Fig. 49. — Diagram of "Ideal Vertebrate," showing the segments of the body, 

 the spinal cord, the notochord, the gill-clefts, the ventral heart. (After Haecket.) 



surface of the body, forming the brain and spinal cord. These 

 arise by the infolding of a skin groove on the dorsal surface of the 

 embryo, (b) Underneath the nerve-cord in the Vertebrate embryo 



