NERVOUS SYSTEM AND BEHAVIOUR 



437 



Giant axons are characteristic of decapods. In Sepia the cell bodies of 

 the third-order giant neurones are scattered through the stellate ganglion; 

 in Loligo they are confined to the posterior lobe (Fig. 10.14). In octopods 

 the same posterior lobe is present and contains cells resembling neurocytes 

 but lacking processes. This lobe, known as the epistellar body, has assumed 

 a neurosecretory role in the octopus, and extirpation leads to loss of 

 muscular tone in the mantle (127, 131). 



Balanoglossids. In most balanoglossids there are conspicuously large 

 neurones in the collar nerve cord. From an anterior group of cell bodies 

 giant fibres pass into the proboscis, while a more posterior group gives 

 rise to axons which run posteriorly to reach the longitudinal musculature 

 of the trunk (Fig. 10.15). Functionally, these giant axons form high-speed 



Proboscis 



Collar nerve cord 



Trunk 



Coelom 

 of collar 



Gill pores 



Dorsal 

 nerve cord 

 of trunk 



Fig. 10.15. Diagrammatic Longitudinal Section through a 

 Balanoglossid, Showing the Giant-Fibre System 

 Cell bodies of giant fibres shown in the collar nerve cord. (After Bullock, 1944.) 



conduction systems for protective avoiding reflexes, and probably serve as 

 final common paths to the muscles involved. Effective stimulation, through 

 these systems, brings about contraction of the proboscis and of the longi- 

 tudinal musculature of the trunk (66). 



Many giant-axon systems still await functional analysis, but studies now 

 available reveal that giant axons of invertebrates are generally involved in 

 mediating protective avoiding reactions. They are of widespread occurrence 

 in burrowing and tubicolous species, in which they are involved in the reflex 

 withdrawal response, whereby the animal jerks back into its shelter when 

 disturbed (polychaetes, brachiopods, balanoglossids). Fast through- 

 conduction pathways in sedentary anemones are concerned with closure 

 of the oral disc. Giant-axon systems of active cephalopods and crustaceans, 

 as we have seen, are concerned with quick escape responses, utilizing jet 

 propulsion and paddle-action. Giant-axon systems conduct quickly, and 

 may produce significant saving of reaction time. It has been estimated 

 that in Myxicola, the giant axon reduces response time to one-half com- 



