248 THE ORIGIN OF THE NERVOUS SYSTEM 



in the body as a whole there is some degree of apico- 

 basal integration, with the apical region acting as 

 receptor and dominant. In certain elongated actinians 

 such as Cerianthus and Harenactis, which live in tubes 

 in the sand with the apical end exposed, this is very 

 clearly the case. 



FUNCTIONAL DIRECTION IN CERTAIN BILATERAL 

 INVERTEBRATES 



The functional direction of the arcs in forms with a 

 synaptic nervous system is generally believed to be 

 determinate and irreversible as regards the arc as a 

 whole, although conduction supposedly occurs in both 

 directions in the axon. The synapse, the region of 

 contact between two neurons, where passage of a cell 

 membrane is involved in conduction of the impulse, is 

 usually regarded as responsible in some way for this 

 functional irreversibility of the arc (see pp. 228-32). 

 The evidence for irreversibility, however, has been 

 obtained chiefly from the higher vertebrates in which 

 it appears to be a characteristic feature of the arc, but 

 it is possible that even in these forms some paths in 

 the central nervous system may be reversible as regards 

 functional direction. If it is true that in the synaptic 

 nervous system the reflex arc is functionally irreversible, 

 then different or opposed functional directions must 

 mean different paths. Certain conditions in some of 

 the lower bilateral invertebrates are of interest as 

 indicating the relation between functional direction and 

 the chief axial gradient, though regarding the actual 

 paths concerned in anterior and posterior directions we 

 know practically nothing. 



