450 THE ORIGIN OF VERTEBRATES 
vertebrate consisted, as far as its gut was concerned, of a prosomatic 
and mesosomatic (branchial) region, close behind which came the 
cloaca and anus. Between the two there was a short metasomatic 
region (possibly pronephric), so that the respiratory chamber did not 
open directly into the cloaca. 
Such an interpretation is, I think, borne out by the study of the 
most ancient forms of fish. In Bothriolepis, according to Patten, 
and in Drepanaspis, according to Traquair, the cloacal region and 
anus follow immediately upon the posterior end of the head-shield, 
i.e. immediately after that region which presumably contained the 
branchie. Similarly, on the invertebrate side, all those forms which 
resembled Limulus must have possessed a very short region between 
the branchial and cloacal parts of the body. The original cloacal 
part of the vertebrate gut may well have been the original cloaca 
of the arthropod, into which its intestine emptied itself, especially 
when we see the tendency of the scorpion group of animals to 
form an accessory cloacal pouch known as the stercoral pouch or 
pocket. 
Again, it is striking to see how, in certain of the scorpion group, 
eg. Thelyphonus and Phrynus, there is a caudal massing of the 
central nerve-cells as well as a cephalic massing, so that their 
central nervous system is composed of a cephalic and caudal brain. 
These two brains are connected together by commissures extending 
the whole length of the body, in which I have been unable to find 
any sign of ganglion-cells. What this caudal brain innervates I 
do not know; it is, I think, a matter worth further investigation, 
especially as there are many indications in the vertebrate that the 
lumbo-sacral region of the cord possesses higher functions than the 
thoracic region. 
The method of formation of the alimentary canal as indicated by 
its innervation is as follows :— 
In front an oral chamber, formed, as already pointed out, by 
the modification of the prosomatic appendages, followed by a 
respiratory chamber, the muscles and branchie of which were 
the muscles and branchiz of the mesosomatic appendages. This 
mesosomatic, or branchial, part was in close contiguity to the cloaca 
and anus, being separated from it only by a short tube formed in the 
metasomatic or pronephric region. 
I imagine that this connection was originally in the form of an 
