CHAPTER XIII 

 THE NOTOCHORD AND ALIMENTARY CANAL 



Relationship between notocliord and gut. — Position of unseginented tube of 

 notochord. — Origin of notocliord from a median groove. — Its function as 

 an accessory digestive tube. — Formation of notocliordal tissue in inverte- 

 brates from closed portions of the digestive tube. — Digestive power of the 

 skin of Ammocoetes. — Fonnation of new gut in Ammocoetes at transforma- 

 tion. — Innervation of the vertebrate gut. — The three outflows of efferent 

 nerves belonging to the organic system. — The original close contiguity of 

 the respiratory chamber to the cloaca. — The elongation of the gut. — 

 Conclusion. 



In t the previous chapters all the important organs of the arthropod 

 have been found in the vertebrate in their appropriate place, of 

 similar structure, and innervated from corresponding parts of the 

 central nervous system. Such comparison is possible only as long- 

 as the ventral and dorsal surfaces of the vertebrate correspond with 

 the respective surfaces of the arthropod, and no reversal is assumed. 

 This method of comparative anatomy is the surest and most 

 certain guide to the relationship between two animals, and when 

 the facts obtained by the anatomical method are so strikingly 

 confirmatory of the paheontological evidence, the combined evidence 

 becomes so strong as to amount almost to a certainty that vertebrates 

 did arise from arthropods in the manner mapped out in previous 

 chapters, and not from a hypothetical group of animals, such as is 

 postulated in the theory of their origin from forms like Balanoglossus. 

 The latter theory derives the alimentary canal of the vertebrate 

 from that of the invertebrate, and finds in the latter the commence- 

 ment of the notochord. In the comparison which I have made the 

 alimentary canal of the invertebrate ancestor has become the tube 

 of the central nervous system of the vertebrate, and there is no sign 

 of a notochord whatever. All the organs of the arthropod have 

 already been allocated ; where the notochord is situated in the 



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