TO 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 
Fig. 1.—ARRANGEMENT OF ORGANS IN THE VERTEBRATE (A) AND ARTHROPOD (B)- 
Al, gut; H, 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 ecelenterate 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 cesophagus, and thus in its highest forms is divided into 
supra-cesophageal and infra-cesophageal nervous masses. These latter 
