CENTRALIZATION AND CEPHALIZATION 145 



t 



and particularly in the sponges, such axiation is not very 

 highly developed. We must expect therefore to find at 

 least some degree of structural or functional axiation, 

 even in the most primitive types of nervous system 

 existent. Even in the sessile coelenterates we do find 

 various indications of nervous axiation and integration 

 (pp. 243-44) and in . the medusae nervous axiation is 

 much more marked. In radial forms, e.g., coelenterates 

 and echinoderms, the different radii are more or less 

 equivalent. There is no indication of any persistent 

 dominance of any particular radial axis either in the medu- 

 sae or in most of the sessile coelenterates. In the medusa, 

 Cassiopea, that particular marginal sense organ which at 

 any given time gives rise to impulses most rapidly, i.e., 

 which has the highest rhythm and is therefore pre- 

 sumably the most active, is for the time being dominant, 

 and similar radial relations apparently exist in hydroids 

 and most actinians. In some actinians, e.g., the Cerian- 

 thidae, there are, however, some indications of a differ- 

 ential or preferential relation among the radii in certain 

 reactions, and in the echinoderms we find this preferential 

 relation or functional dominance of certain radii, in all 

 degrees of development even involving marked structural 

 bilaterality of the body wall in certain groups. 



Nervous differentiation is not coincident with the 

 beginnings of axiation in animals but represents a later 

 stage in the development of axiation, consequently the 

 nervous system shows some degree of axiate pattern even 

 in its most primitive forms; in other words, wherever 

 differentiation progresses far enough to give rise to any 

 distinguishable nervous structure some degree of struc- 

 tural or functional axiation or centralization is probably 



