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 

 branchia3. 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, 

 e.g. 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 branchise of which were 

 the muscles and branchise 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 



