ROUND-MOUTHS. 105 



described as a peculiar form of fish under the name of 

 Ammocoetes. By a further metamorphosis this blind and 

 toothless Ammocoetes is transformed into the Lamprey with 

 eyes and teeth (Petromyzon). 141 



Summing up all these peculiarities in the structure and 

 embryology of the Round-mouths, we may assert that the 

 oldest Skulled Animals, or Craniota, diverged in two lines ; 

 one of these lines has continued up to the present time 

 but little modified; it is represented by the Cyclostoma, 

 or Monorhina, forming a collateral line which has made 

 but little progress, but has remained at a very low stage of 

 development. The other line, the direct line in the pedigree 

 of the Vertebrates, advanced in a straight line to the Fishes, 

 and by new adaptations attained many important improve- 

 ments. 



In order rightly to appreciate the phylogenetic signi- 

 ficance of interesting remnants of primseval groups of 

 animals, such as the Round-mouths, it is necessary to study 

 minutely their various peculiar characters philosophically 

 and with the aid of Comparative Anatomy. A careful 

 distinction must be drawn between the hereditary cha- 

 racters which have been accurately transmitted to the 

 present day by heredity from common, primaeval ancestors, 

 now extinct, on the one hand ; and, on the other, those 

 special adaptive peculiarities which the existing remnant 

 of that primaeval group have, in the course of time, gained 

 secondarily by adaptation. To the latter class belong, 

 for example, in the Round-mouths, the peculiar formation 

 of the single nostril and the round sucking mouth ; as 

 well as special structural arrangements of the epidermis 

 and the pouch-shaped gills. But, on the other hand, to the 



