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THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 37 
is one, if not the most striking, of the peculiarities which distinguish 
the vertebrate ; a tubular central nervous system such as that of the 
vertebrate is totally unlike any other nervous system, and the very 
fact that the two nervous systems of the vertebrate and arthropod 
are so similar in their nervous arrangements, makes it still more 
extraordinary that the nervous system should be grouped round a 
tube in the one case and not in the other. 
Now, in the arthropod the cesophagus leads directly into the 
stomach, which is situated in the head-region, and from this a straight 
intestine passes directly along the length of the body to the anus, 
where it terminates. The relations of mouth, oesophagus, alimentary 
canal, and nervous system in these animals are represented in the 
diagram (Fig. 3). 
Any tube, therefore, such as that of the infundibulum, which 
would represent the cesophagus of such an animal, must have opened 
into the mouth on the ventral side, and into the stomach on the 
dorsal side, and the lining epithelium of such an cesophagus must 
have been continuous with that of the stomach, and so of the whole 
intestinal tract. 
Supposing, then, the animal is not turned over, but that the dorsal 
side still remains dorsal and ventral ventral, then the original mouth- 
opening of the esophagus must be looked for on the ventral surface 
of the vertebrate brain in the region of the pituitary body or hypo- 
physis, and on the dorsal side the tube representing the cesophagus 
must be continuous with a large cephalically dilated tube, which 
ought to pass into a small canal, to run along the length of the body 
and terminate in the anus. 
This is exactly what is found in the vertebrate, for the infun- 
dibular tube passes into the third ventricle of the brain, which forms, 
with the other ventricles of the brain, the large dilated cephalic 
portion of the so-called nerve tube, and at the junction of the medulla 
oblongata and spinal cord, this dilated anterior part passes into the 
small, straight, central canal of the spinal cord, which in the embryo 
terminates in the anus by way of the neurenteric canal. If the 
animal is regarded as not having been turned over, then the con- 
clusion that the infundibulum was the original cesophagus leads 
immediately to the further conclusion that the ventricles of the verte- 
brate brain represent the original cephalic stomach, and the central 
canal of the spinal cord the straight intestine of the arthropod ancestor. 
