400 



G. ADRIAN HORRIDGE 



Siphonozooids 



ndece 



Fig. 1. A colony o'[ Heteroxenia fiiscescens (Alcyonacea) of the Red Sea, showing 

 the numerous autozooids which are continually and independently spon- 

 taneously active except when inhibited by a specialized nerve net which runs 

 over the whole colony. From Horridge (1956b). 



each feeding motor impulses into a through-conducting nerve net which 

 co-ordinates the symmetrical beat of the muscles over the whole bell. In the 

 jellyfish these centres are separate ganglia, in which incidentally, neuropile 

 is abundant in the first ganglia which appear in the animal kingdom. There 

 is, in addition, a second nerve net which forms an input to these gangUa 

 but over the surface of the bell, where they overlap, there are no physiological 

 junctions between the two nerve nets. In some species a single shock applied 

 to this second net immediately inhibits the spontaneity of all the marginal 

 ganglia; in other species an impulse in the second net initiates an efferent 

 impulse or accelerates the rhythm. Inhibition, rather than acceleration, of the 

 rhythm of the ganglion is significant here in showing that the modulating 

 pathways are widespread, because inhibition of a number of linked pace- 

 makers shows that they must all have been acted on, but acceleration can be 

 achieved by acting on one of them alone. 



The main motor output of the jellyfish ganglion is a nerve net, the so- 

 called giant fibre net, which carries a nerve impulse at each beat of the bell. 

 The pacemaker, lying in the ganglion and physiologically within this motor 

 net, is modulated by a variety of stimuli, including light and mechanorecep- 

 tors, in or near the ganglion, in addition to the afferent supply from the prim- 

 ary nerve net. There seems no reason to prefer a set of anatomically addressed 

 connexions of these various inputs rather than a general sensitivity of the 

 pacemakers to transmitter substances from non-addressed afferent neurons. 



If only one efferent path were known from the jellyfish ganglion there 

 would not be the necessity for an addressed system within it, except to dis- 



