Evolution 



lamprey and the hag are illustrated in Figs. 66 and 67, and some 

 of the main anatomical features of the group are shown in Figs. 

 63, 68, and 69. 



There remain, even after the most thorough investigation of 



FIG. 66. The Lamprey Petromyzon. 



Balanoglossus, the lancelet, and the round-mouths, some questions 

 with regard to the origin of the vertebrates that are still un- 



FIG. 67. The Hag-fish Myxine. 



answered. It must, however, be regarded as an extremely 

 fortunate circumstance that representatives have come down to us 

 of the three ancient groups to which these three types respectively 

 belong. This is the more fortunate in that the groups in question 



88 



