THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM O61 
resemblance to that of the vertebrate. In the former case a trans- 
formation also takes place, a marvellous transformation, characterized 
_ by two most striking facts. On the one hand, the resulting animal 
is more like a higher vertebrate, for, by the formation of new 
cartilages, its cranial skeleton is now comparable with that of the 
higher forms, and the beginnings of the spinal vertebrae appear; by 
the increased formation of nervous material, its brain increases in 
size and complexity, so as to compare more closely with higher 
vertebrate brains; its eyes become functional, and its branchie are 
so modified, Simultaneously with the formation of the new alimentary 
canal in the cranial region, that they now surround branchial pouches 
which are’ directly comparable to those of higher vertebrates. On 
the other hand, the transformation process is equally characterized 
by the throwing off of tissues and organs, one and all of which are 
comparable in structure and function with corresponding structures in 
the Arthropoda—the thyroid of the Ammoceetes, the tentacles, the 
muco-cartilage, the tubular muscles, all these structures, so striking 
in the Ammoceetes stage, are got rid of at transformation. Here is 
the true clue. Here, in the throwing off of invertebrate characters, 
and the taking on of a higher vertebrate form, especially a higher 
brain, not a lower one, Petromyzon proclaims as clearly as is possible 
that it is not a degenerate elasmobranch, but that it has arisen from 
Ammocecetes-like ancestors, even though Myxine, Amphioxus, and 
the tunicates be all stages on the downward grade from those same 
Ammoceetes-like ancestors. 
As to the eyes, they are functional in the adult form and as service- 
able as in any fish. There is no sign of degeneracy; it is only possible 
to speak of a retarded development which lasts through the larval stage. 
CoMPARISON OF BRAIN OF AMMOCG@TES WITH THAT OF AN 
ARTHROPOD, 
Seeing that the steady progress of the development of the central 
nervous system is the most important factor in the evolution of 
animals, it follows that of all organs of the body, the central nervous 
system must be most easily comparable with that of the supposed 
ancestor. I will, therefore, start by comparing the brain of 
Ammoccetes with that of arthropods, especially of Limulus and of 
the scorpion-group. 
