448 THE BIOLOGY OF MARINE ANIMALS 



manubrium towards a stimulated tentacle, and general contraction of the 

 stimulated and other tentacles. Movements of the manubrium are mediated 

 by a radial conducting system, exciting the radial muscles of the manub- 

 rium, whereas tentacular contraction is co-ordinated by a through-con- 

 ducting pathway in the marginal nerve. The two conducting systems serv- 

 ing circular and radial muscles are independent of each other in Geryonia, 

 and transmission can occur simultaneously in each. In Aequorea, on the 

 contrary, the feeding response stops swimming, inhibition depending on 

 some process in the ganglionated ring nerves (56, 57). 



Reflex and Integrative Activities of Invertebrate 

 Central Nervous Systems 



Nerve-nets mediate reflex activity in so far as this is regarded as a 

 sequence of events involving receptor, connecting neurones and effector. 

 At higher levels a central nervous system (ganglia and nerve cords) is 

 involved in the arc, and forms a node from which impulses, originating in 

 peripheral receptors, are reflected peripherally to the effector organs. In 

 bilaterally symmetrical animals such systems typically comprise cephalic 

 ganglia and ganglionated cords, to which afferent fibres proceed from the 

 periphery, and from which efferent fibres proceed to effectors. A simple 

 reflex, as a theoretical concept, would involve a temporal sequence of 

 effective environmental stimulus, discharge of receptor, excitation of 

 efferent neurones, and activation of effector. Probably no reflex is as 

 simplified as this. Even in the nerve-net of lower metazoans, excitation of 

 one set of effectors may be attended by reciprocal inhibition of antagonists. 

 Reflexes, then, involve co-ordinative activity of systems of effectors; 

 central neurones, by virtue of their functional organization, regulate the 

 relative activities of the various effectors. 



Animals which have become elongated and bilaterally symmetrical have 

 the sense organs concentrated at the anterior end. This is the region chiefly 

 exposed to environmental stimuli as the animal advances, or when it pro- 

 trudes its anterior region from the shelter in which it is lodged. Pari passu, 

 with elaboration of sensory fields, nerve centres have become concentrated 

 in anterior regions and serve as relay centres for the enlarged peripheral 

 sensory fields. Segmentation involves duplication of ganglionic regions 

 along the length of the animal, each ganglion acting as a relay centre for 

 local reflexes. The central nervous system may be regarded as a unit 

 governing integrated activities of the whole organism, whereas subsidiary 

 metameric centres control regional activity. 



Reflex Activity. Reflex functioning in invertebrates has received noth- 

 ing like the same attention as the spinal reflexes of vertebrates, nor may 

 the same integrated pattern of functional activities be expected because of 

 the diversity of neural organization in different phyla. 



All animals show reflexes, even those whose nervous systems are built 

 entirely on the plan of the nerve-net. Interest in these activities is focused 

 on the basal neural mechanisms responsible for each reflex act, and their 



