ORIGIN OF TEE NERVOUS SYSTEM 259 



merit of cellular elements as is seen in this animal, i. e., the ganglion- 

 cells are on the side of the cord next the exterior, and the neuropile and 

 nerve-fibers next the interior. 



The chief fundamental point of difference in the nervous systems of 

 the annelids and arthropods consists in the great number of long 

 neurones in the latter as compared with the former. In the crab, as 

 demonstrated by Bethe (1897), many of the primary sensory neurones 

 extend over half the length of the ventral cord instead of being limited 

 to a few segments as in the earthworm, and the same is true of the 

 primary motor neurones. Moreover, the association neurones have 

 shown an extensive growth. Although in the crab there are some 

 neurones limited to one or two segments, as is the rule in the earth- 

 worm, the great majority extend over many segments and often through 

 the whole length of the nervous system. In this way the central 

 nervous organs of these animals are locked together much more closely 

 than are those in the worm and exhibit consequently in their physiology 

 a unity that the worms do not possess. This nervous unity, moreover, 

 has developed to such a degree in the higher arthropods that we may 

 with reason ascribe to such animals as the insects a primitive form of 

 intellectual life not unlike that found in the vertebrates. The struc- 

 tural basis for this seems to me to be foreshadowed in the few long 

 neurones of the worm which, as I have just pointed out, come to be the 

 common type in the arthropods. The type of central nervous system 

 with long neurones also characterizes the other higher invertebrates 

 such as the mollusks, etc. 



The central nervous system of the vertebrates and of certain other 

 closely allied forms like the tunicates, is usually put in strong contrast 

 with that of the higher invertebrates. The most striking feature in 

 this contrast is the fact that the vertebrate nervous system is tubular 

 and the invertebrate solid. As is well known, the central nervous 

 organs in vertebrates develop from an ectodermic tube that has been 

 infolded from the median dorsal surface of the animal. This simple 

 nerve-tube with nervous connections, but otherwise almost unmodified, 

 exists to-day in that primitive vertebrate amphioxus. In the higher 

 vertebrates the posterior portion of this tube becomes uniformly thick- 

 ened and forms the spinal cord, the central canal of which gives evi- 

 dence of its tubular nature. The anterior portion undergoes still more 

 profound changes than the posterior part in that its wall thickens very 

 differently in different regions and expands in several lobe-like out- 

 growths, giving rise thus to the brain whose ventricles represent the 

 original cavity of the nerve-tube. 



Notwithstanding the striking difference between the central nervous 

 organs of vertebrates and invertebrates, they show certain fundamental 

 similarities and the first of these has to do with the distribution of 



