CENTRALIZATION AND CEPHALIZATION 149 



associated and localized in a definite way in relation to 

 regions of high activity. In this connection it may also 

 be suggested that certain ganglia outside the central 

 nervous system in other forms arise in connection with 

 secondary regions of high activity. 



THE "STEPLADDER' : TYPE OF NERVOUS SYSTEM IN 



THE INVERTEBRATES 



The relation of the central nervous system to the 

 symmetry gradients is definite and clear except in some 

 of the lower invertebrates. In most of the bilateral 

 invertebrates in which longitudinal nerve cords form 

 part of the central nervous system, viz., most flatworms, 

 annelids, and arthropods, the postcephalic longitudinal 

 portions of the central nervous system are ventral, in 

 the vertebrates dorsal, and except in some of the lower 

 invertebrates they arise as a bilateral organ at or near 

 the median line. Various lines of experiment and 

 observation indicate that median and ventral in most 

 bilateral invertebrates and median dorsal in the verte- 

 brates, represents the high region of the symmetry 

 gradients and is, at least during earlier stages of develop- 

 ment, a region of more intense physiological activity 

 than the regions on either side of it (pp. 26, 58). 



In some of the lower invertebrates, however, the 

 relation between the longitudinal nerve cords and the 

 axial gradients is not so clear. In Planaria, for example, 

 and in most other Turbellaria the nerve cords lie some 

 distance lateral to the median line on each side (Fig. 19, 

 p. 122), in the cestodes the chief nerve cords are near the 

 lateral margins of the proglottides, and in the nemer- 

 teans also the nerve cords are more lateral than median. 



