36 THE ORIGIN OF VERTEBRATES 
it be supposed that the arthropod arose from the segmented worm, 
the caterpillar bears out the suggestion better than the adult imago. 
If it be supposed that the tunicate arose from a stock allied to the 
vertebrate, it is because of the peculiarities of the larva that such a 
supposition is entertained. So, too, if it be supposed that the fish 
arose from a member of the arthropod group, the larval form of the 
fish is most likely to give decisive information on the point. 
For all these reasons the lowest form of fish to be investigated, 
in the hopes of finding out the nature of the earliest formed fish, is 
not Amphioxus, but Ammoceetes, the larval form of the lamprey—a 
form which, as I hope to satisfy my reader after perusal of subse- 
quent pages, more nearly resembles the ancient Cephalaspidian fishes 
than any other living vertebrate. 
COMPARISON OF CENTRAL NERVOUS SYSTEMS OF VERTEBRATE AND 
ARTHROPOD WITHOUT REVERSAL OF SURFACES. 
So far different lines of investigation all point to the origin of the 
vertebrate from arthropods, the group of arthropods in question being 
now extinct, the nearest living representative being Limulus; also to 
the fact that of the two theories of the origin of vertebrates, that 
one which is based on the resemblance between the central nervous 
systems of the Vertebrata and the Appendiculata (Arthropoda and 
Annelida) is more in accordance with this evidence than the other, 
which is based mainly on the supposed possession of a notochord 
among certain animals. 
How is it, then, that this theory has been discredited and lost 
ground? Simply, I imagine, because it was thought to necessitate 
the turning over of the animal. Let us, then, again look at the 
nervous system of the vertebrate, and see whether there is any such 
necessity. 
As previously mentioned, the comparison of the two central 
nervous systems showed such close resemblances as to force those 
anatomists who supported this theory to the conclusion that the 
infundibular tube was in the position of the original cesophagus ; 
they therefore looked for the remains of a mouth opening in the 
dorsal roof of the brain, but did not attempt to explain the extra- 
ordinary fact that the infundibular tube is only a ventral offshoot 
from the tube of the central nervous system. Yet this latter tube 
