CONCLUSION. 289 



CONCLUSION. 



From the facts that have been recorded and the consid- 

 erations that have been urged in these pages, it would 

 follow that one of the chief factors in the evolution of the 

 Vertebrates has been the concentration of the central 

 nervous system along the dorsal side of the body (in 

 contrast to the position of the longitudinal nerve-cord of 

 Annelids, etc., along the ventral or locomotor surface), and 

 its conversion into a hollow tube. If it be admitted that 

 the hypophysis became evolved in connexion with a func- 

 tional neuropore, it is obviously a structure which has 

 arisen within the limits of the Vertebrate phylum, and can, 

 therefore, have no representative in the typical Invertebrate 

 organisation. It has been suggested by Adam Sedgwick 

 and VAN WijHE that the original function of the central 

 canal of the spinal cord was to promote the respira- 

 tion (oxygenation) of the tissue of the central nervous 

 system, water entering by the neuropore, and passing out 

 through the posterior neurenteric canal. 



It is not so easy to form a conception as to the prime 

 origin of the other two cardinal characteristics of a 

 Vertebrate (Chordate) ; namely, gill-slits and notochord. 



As to the origin of gill-slits, it has been suggested inde- 

 pendently by Hakmer and Brooks, that they arose at first 

 not so much to perform the direct function of respiration, 

 as to carry away the bulk of the water which constantly 

 entered the mouth with the food, so as to avoid the neces- 

 sity and discomfort of the never-ceasing flow of water 

 through the entire length of the alimentary canal. In 

 Cephalodiscus, for example, the luxuriant branchial plumes 

 must be sufficient for the respiration of the minute animal, 



