246 THE ORIGIN OF THE NERVOUS SYSTEM 



regards the coelenterates, the work of Parker on actinians 

 and Corymorpha and the work of others on medusae 1 

 indicate that in these forms nervous transmission is 

 highly diffuse and may occur in all directions from most 

 or all regions of the body in the actinians and at least 

 of the subumbrellar surface in the medusae. The 

 experimental data do not indicate any differences in 

 conductivity or range in any direction with respect to 

 the body axis. I believe, however, that crucial experi- 

 ments bearing upon this point have not yet been per- 

 formed. Parker's experiments on the actinians, for 

 example, are not adapted to show whether axial differ- 

 ences in irritability, conductivity, and range are present 

 or not. For this purpose stimuli graded in intensity 

 would be necessary, and aquatic forms are not favorable 

 material for work of that kind. Mayer and Harvey 

 have shown that impulses may apparently continue to 

 travel indefinitely without any indication of decrement 

 in circular strips of the medusa, Cassiopea, from which 

 the sense organs have been removed, but they have not 

 determined whether there is any difference in conduc- 

 tivity in the direction away from and toward the sense 

 organs. There can be no doubt that in the medusae 

 the sense organs are the most irritable regions and 

 therefore in nature usually the initiators of impulses. 



In the free-swimming medusae the specialization of 

 both receptors and effectors is far more advanced than 

 in the sessile coelenterates, and the whole receptor- 

 conductor-effector mechanism is much more highly 



' Kimcr (1878), Harvey (191 2), Mayer (1906, 1908, 1916), Parker 

 (1916, 191 711, b, c, d,f), Parker and Titus (1916), Romanes (1877, 1878, 

 1893)- 



