IO 



THE ORIGIN OF VERTEBRATES 



cartilaginous skeleton from a group possessing an external skeleton 

 of a calcareous or chitinous nature, but rather the difficulty caused by 

 the fundamental difference of arrangement of the important internal 

 organs, especially the relative positions of the central nervous system 

 and the digestive tube. 



Now, if we take a broad and comprehensive view of the inver- 

 tebrate kingdom, without arguing out each separate case, we find that 



D 



B 



Fig. 1. — Arrangement of Organs in the Vertebrate (A) and Arthropod (B)' 

 Al, gut; IT, heart; C.N.S., central nervous system; V, ventral side; D, dorsal side. 



it bears strongly the stamp of a general plan of evolution derived 

 from a co;lenterate animal, whose central nervous system formed a 

 ring surrounding the mouth. Then when the radial symmetry was 

 given up, and an elongated, bilateral, segmented form evolved, the 

 central nervous system also became elongated and segmented, but, 

 owing to its derivation from an oral ring, it still surrounded the 

 mouth-tube, or oesophagus, and thus in its highest forms is divided into 

 supra- oesophageal and infra-oesophageal nervous masses. These latter 



