CHAPTER XIII 
THE NOTOCHORD AND ALIMENTARY CANAL 
Relationship between notochord and gut.—Position of unsegmented tube of 
notochord.—Origin of notochord from a median groove.—Its function as 
an accessory digestive tube.—-Formation of notochordal tissue in inverte- 
brates from closed portions of the digestive tube—Digestive power of the 
skin of Ammocotes.—Formation of new gut in Ammoceetes 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 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 paleontological 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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