64 THE ORIGIN OF VERTEBRATES 
A review of the animal kingdom as a whole leads to the conclusion that the 
upward development of animals from an original colenterate stock, in which 
the central nervous system consists of a ring of nervous material surrounding 
the mouth, has led, in consequence of the elaboration of the central nervous 
system, to a general plan among the higher groups of invertebrates in the topo- 
graphical arrangement of the important organs. The mouth is situated ventrally, 
and leads by means of the cesophagus into an alimentary canal which is situated 
dorsally to the central nervous system. Thus the esophagus pierces the central 
nervous system and divides it into two parts, the supra-csophageal ganglia 
and the infra-cesophageal ganglia. This is an almost universal plan among 
invertebrates, but apparently does not hold for vertebrates, for in them the 
central nervous system is always situated dorsally and the alimentary canal 
ventrally, and there is no piercing of the central nervous system by an esophagus. 
Yet a remarkable resemblance exists between the central nervous system of 
the vertebrate and that of the higher invertebrates, of so striking a character as 
to compel one school of anatomists to attempt the derivation of vertebrates 
from annelids. Now, the central nervous system of vertebrates forms a hollow 
tube, and a diverticulum of this hollow tube, known as the infundibulum, passes 
to the ventral surface of the brain in the very position where the cesophagus 
would have been if that brain had belonged to an annelid or an arthropod. 
This school of anatomists therefore concluded that this infundibular tube 
represented the original invertebrate cesophagus which had become closed and 
no longer opened into the alimentary canal owing to the formation of a new 
mouth in the vertebrate. As, however, the alimentary canal of the vertebrate 
is ventral to the central nervous system, and not dorsal, as in the invertebrate, 
it follows that the remains of the original invertebrate mouth into which the 
cesophagus (in the vertebrate the infundibular tube) must have opéned must be 
searched for on the dorsal side of the vertebrate; and so the theory was put 
forward that the vertebrate had arisen from the annelid by the reversal of 
surfaces, the back of the one animal becoming the front of the other. 
The difficulties in the way of accepting such reversal of surfaces have proved 
insuperable, and another school has arisen which suggests that evolution has 
throughout proceeded on two lines, the one forming groups of animals in which 
the central nervous system is pierced by the food-channel and the gut therefore 
lies dorsally to it, the other in which the central nervous system always lies 
dorsally to the alimentary canal and is not pierced by it. In both cases the 
highest products of the evolution have become markedly segmented animals, in 
the former, annelids and arthropods; in the latter, vertebrates. The only 
evidence on which such theory is based is the existence of low forms of animals, 
known as the Enteropneusta, the best known example of which is called 
Balanoglossus ; they are looked upon as aberrant annelid forms by many 
observers. 
This theory does not attempt to explain the peculiarities of the tube of the 
vertebrate central nervous system, or to account for the extraordinary resemblance 
between the structure and arrangement of the central nervous systems of 
vertebrates and of the highest invertebrate group. 
Neither of these theories is satisfactory or has secured universal acceptance. 
The problem must be considered entirely anew. Whatare the guiding principles 
in this investigation ? 
