648 COMPARATIVE ANATOMY 



Strongly resemble the paleostracan forms. The limulus or king-crab is a 

 living representative of the paleostracan ancestors of fishes. From such a 

 form vertebrates may have evolved without the necessity of reversal of 

 dorsal and ventral sides. This is possible if we assume that the infundibu- 

 lar tube of the vertebrate brain represents the esophagus of the arachnid, 

 that the vertebrate forebrain is the supraesophageal ganglion, the crura 

 cerebri the esophageal commissures, the remainder of the brain the 

 subesophageal ganglia. The ventricles of the brain correspond to the 

 cephahc stomach of the arthropod. In the arthropod, the cephalic 

 stomach leads directly into the straight narrow intestine; in the vertebrate 

 the fourth ventricle leads into the straight narrow canal of the spinal cord. 



"In the arthropod the intestine terminates in the anus; in the verte- 

 brate embryo the canal of the spinal cord terminates in the anus by way 

 of the neurenteric canal. Keep the animal unreversed, and immediately 

 the whole mystery of the tubular nature of the central nervous system 

 is revealed, for it is seen that the nervous matter, which corresponds bit 

 by bit with that of the arthropod, has surrounded to a greater or less 

 extent and amalgamated with the tube of the arthropod alimentary canal, 

 and thus formed the so-called central nervous system of the vertebrate. 



"The manner in which the nervous material has invaded the walls 

 of the tube is clearly shown both by the study of the comparative anatomy 

 of the central nervous system in the vertebrate, and also by its develop- 

 ment in the embryo. 



"This theory implies that the vertebrate alimentary canal is a new 

 formation necessitated by the urgency of the case, and, indeed, there 

 was cause for urgency, for the general plan of the evolution of the inverte- 

 brate from the coelenterate involved the piercing of the anterior portion 

 of the central nervous system by the esophagus, while, at the same time, 

 upward progress meant their .development; brain development meant 

 concentration of nervous matter at the anterior end of the animal, with 

 the result that in the highest scorpion and spider-like animals, the brain 

 mass has so grown round and compressed the food-tube that nothing but 

 fluid pabulum can pass through into the stomach; the whole group have 

 become blood-suckers. These kinds of animals, the sea scorpions, were 

 the dominant race when vertebrates first appeared; here in the natural 

 competition among members of the dominant race the difficulty must 

 have become acute. Further upward evolution demanded a larger and 

 larger brain with the ensuing consequence of a greater and greater dif- 

 ficulty of food-supply. Nature's mistake was rectified and further 

 evolution secured, not by degeneration in the brain region, for that 

 means degradation and not upward progress, but by the formation of a 

 new food-channel, in consequence of which the brain was free to develop 

 to its fullest extent. Thus the great and mighty kingdom of the verte- 



