THE REFLEX ARC 243 



than in the sessile coelenterates. It may be pointed 

 out, however, that in the sessile coelenterates there is 

 normally a reversal of polarity during development. 

 The planula possesses a well-marked apico-basal gra- 

 dient, but as the period of attachment approaches this 

 gradually disappears, and finally it is the original apical 

 end which becomes attached, and the sessile individual 

 arises as a new axial gradient, essentially a bud, at the 

 original basal end. Moreover, in these sessile forms 

 axiation is never very highly developed. Again in the 

 sponge, which Parker regards as representing the primi- 

 tive condition, the early developmental motile stage 

 possesses a definite physiological axis, i.e., a primitive 

 physiological gradient exists, but this disappears more 

 or less completely during the later development, and 

 the sessile adult exhibits an extremely low degree of 

 physiological axiation and integration, so that practically 

 nothing but local or unicellular arcs exist. But the 

 sessile sponge and the sessile coelenterate develop from 

 motile forms and the almost anaxiate condition oi the 

 adult is not primitive but secondary. Looking at the 

 matter from this point of view, it is scarcely possible to 

 agree with Parker, either that the contractile cells of 

 the sponge are pure effectors or that the conditions in 

 the sponges and sessile coelenterates represent the first 

 step in the evolution of the receptor-conductor-effector 

 system in free-living forms. They appear rather as 

 special cases in which a form originally free-swimming 

 and axiate has become sessile and almost anaxiate. 

 Corresponding to the very low degree of integration in the 

 adult sponge no distinguishable nervous structure is 

 present, and in the unicellular or local arcs only the 



