THE NERVOUS SYSTEM 



475 



If we take the subcutaneous nerve-net of coelenterates as an early 

 stage in the evolution of the central nervous system and the neurosensory 

 cells as the beginnings of sensory nerves, the primitive ganglion cells are 

 the original association and motor cells. From such beginnings it is not 

 difficult to derive the complex nervous systems of the higher animals. 



The fiatworms show a distinct advance above the coelenterate stage. 

 In them, the nerve net is partly aggregated into two or more paired longi- 

 tudinal cords or connectives, which unite at the anterior end of the worm, 

 in close association with pigmented eye-spots, to form the beginnings of a 

 brain. (Fig. 392) 



Morphologists are inclined to derive the paired lateral nerve cords of 

 fiatworms directly from the subumbrellar ganglionic ring of a medusa. 



A. tOWER FLATWORM B. HIGHER FLATWORM C. ARTHROPOD D. VERTEBRATE 



Fig. 392. — Diagrams of the nervous system of A, Lower Flatworm, B, Higher 

 Flatworm, C, Arthropod, and D, Vertebrates. The higher nerve centers are cross- 

 hatched. The sympathetic cords are indicated by dotted lines. Many morphologists 

 assume that the figures represent a phylogenetic series. Such a view, however, which 

 assumes that annelids are ancestors of vertebrates, meets with very serious, if not 

 insuperable, diffictilties. See Chapter 15. (Redrawn after Stempell.) 



Like the subumbrellar ring, the nerve cords of fiatworms consist of nerve 

 fibers associated with clusters of primitive ganglion cells. Kappers 

 explains the concentration of nervous material in the anterior brain as a 

 result of the great exposure of the head to stimuli. Unfortunately, such 

 an hypothesis is unsupported by experimental evidence. (Fig. 393) 



In the simple flatworm Planocera, there is a single pair of nerve cords. 

 The number increases in other forms, and the cords may be dorsal and 

 ventral as well as lateral. This fact is important in its bearing upon the 

 development of the nervous systems of higher animals, which in general 

 are assumed to have evolved from fiatworm-like ancestors. For the 

 presence of both dorsal and ventral nerve cords in fiatworms makes it 

 possible to derive annelids and arthropods from fiatworms in which the 

 ventral cords become the dominant nervous centers, and to derive verte- 

 brates from fiatworms in which the dorsal cords become predominant. 

 The dorsal nerve cord of vertebrates therefore need not have been derived 

 from the ventral cord of annelids by the inversion of the worm. It has 



