THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 11 
nervous masses are of necessity ventral to the digestive tube, because 
the mouth of the ccelenterate is on the ventral side. The striking 
characteristic, then, of the invertebrate kingdom is the situation of a 
large portion of the central nervous system ventrally to the alimentary 
canal and the piercing of the nervous system by a tube—the veso- 
phagus—leading from the mouth to the alimentary canal. The 
equally striking characteristic of the vertebrate is the dorsal position 
of the central nervous system and the ventral position of the ali- 
mentary canal combined with the absence of any piercing of the 
central nervous system by the cesophagus. 
So fundamentally different is the arrangement of the important 
organs in the two groups that it might well give rise to a feeling of 
despair of ever hoping to solve the problem of the Origin of Verte- 
brates; and, to my mind, this is the prevalent feeling among 
morphologists at the present time. Two attempts at solution have 
been made. The one is associated with the name of Geoffrey St. 
Hilaire, and is based on the supposition that the vertebrate has 
arisen from the invertebrate by turning over on its back, swimming 
in this position, and so gradually converting an originally dorsal 
surface into a ventral one, and vice versdé; at the same time, a new 
mouth is supposed to have been formed on the new ventral side, 
which opened directly into the alimentary canal, while the old 
mouth, which had now become dorsal, was obliterated. 
The other attempt at solution is of much more recent date, and is 
especially associated with the name of Bateson. It supposes that 
bilaterally symmetrical, elongated, segmented animals were formed 
from the very first in two distinct ways. In the one case the diges- 
tive tube pierced the central nervous system, and was situated dorsally 
to its main mass. In the other case the segmented central nervous 
system was situated from the first dorsally to the alimentary canal, 
and was not pierced by it. In the first case the highest result of 
evolution led to the Arthropoda; in the second case to the Vertebrata. 
Neither of these views is based on evidence so strong as to cause 
universal acceptance. The great difficulty in the way of accepting 
the second alternative is the complete absence of any evidence, either 
among animals living on the earth at the present day or among those 
known to have existed in the past, of any such chain of intermediate 
animal forms as must, on this hypothesis, have existed in order to 
link together the lower forms of life with the vertebrates. 
