NERVOUS SYSTEM AND BEHAVIOUR 



427 



Conduction in the balanoglossid nerve plexus is diffuse and decremental, 

 and pieces of the body wall show a high degree of autonomy. These features 

 are in agreement with the structural picture of diffusely arranged sensory, 

 intermediate and motor neurones. Decremental conduction, as in coelen- 

 terates, would appear to involve facilitation for overcoming synaptic 

 resistance in interrupted pathways. Cords and other nerve tracts are 

 distributive in function, rather than integrative. 



Diffuse conduction is revealed by continued transmission of excitation 

 following cuts in various directions through the trunk and proboscis 

 (Balanoglossus, Saccoglossus). The three main regions of the body possess 

 mechanisms for nervous reflexes, and show local muscular responses to 

 tactile and photic stimuli. The sub-epithelial plexus is the site of this diffuse 





Fig. 10.6. Facilitation in a Strip-preparation of the 

 Scyphomedusan Rhopilema 



Figures refer to intervals between shocks in seconds. (From Bullock (17).) 



nervous transmission in the absence of longitudinal nerve cords. Dorsal 

 and ventral nerve cords form through-conduction systems, and when the 

 cords are cut the shortening reflex is impaired, probably owing to inter- 

 ruption of giant-axon pathways (vide p. 437). 



Locomotion in Saccoglossus and other enteropneusts is accomplished 

 largely by ciliary activity, aided by peristaltic movements of the proboscis. 

 The cilia on the trunk appear to be under nervous control, and peristaltic 

 contractions are regulated by the dorsal nerve cord. Autonomous nervous 

 activity is centred to a large extent in the proboscis, which is responsible 

 for much of the neural drive of the animal. Such activity is not localized 

 in any one part of the proboscis, but the dorsal nerve cord is necessary for 

 longitudinal conduction (66). 



Contractions of the body wall of solitary sea-squirts (Phallusia, etc.) 

 show well-marked facilitation under repetitive stimulation (Fig. 10.7). 

 What part the nerve-net plays in the mediation of these responses still 

 awaits clarification (58, 59). 



