NERVOUS SYSTEM AND BEHAVIOUR 449 



integration into the total behaviour of the animal. The closure reflex of the 

 sea-anemone has probably been the most intensively studied coelenterate 

 reflex. This response (described on p. 385) involves endodermal stimulation, 

 transmission along a specialized through-conduction system in the longi- 

 tudinal mesenteries, facilitation at the junction between nerve-net and oral 

 sphincter, and contraction of the latter {Calliactis). With graded and con- 

 trolled mechanical stimuli, evocation of the response is found to depend 

 upon tensile stimulation of the endodermal surface. Several brief "prods" 

 above threshold produce a series of impulses and a facilitated response. 

 Maintained pressure initiates several pulses, but soon loses its effectiveness 

 owing to adaptation, mechanical and sensory. 



In the sea anemone Calliactis it appears as if the entire endodermal wall 

 of the column were acting as one integral tensile receptor, deformation 

 of any part of which could lead to discharge in the through-conduction 

 nerve-net and reflex contraction of the sphincter. Oral disc and tentacles 

 are far more sensitive than the column, and a very slight mechanical 

 stimulus is sufficient to produce a localized feeding or rejection response 

 in the former structures (86). 



The peristaltic contractions responsible for swimming movements in 

 flat worms are reflexly controlled (Yungia). The brain is a sensory centre of 

 low threshold, impulses from which bring the locomotory mechanisms into 

 activity. After decapitation and removal of the brain the animal becomes 

 quiescent, owing to the higher threshold of remaining receptor pathways. 

 The neuromuscular mechanism for co-ordinated swimming movements 

 is still present, however, and swimming can be induced by mechanical 

 stimulation of the cut anterior end of the animal (70). 



Ambulation in polychaetes is produced by peristaltic waves of the body 

 musculature and movements of the parapodia. The transmission of ex- 

 citation responsible for these waves takes place in the nerve cord. Patterns 

 of ambulation are centrally controlled, but can be evoked by appropriate 

 reflex stimulation. In the lugworm Arenicola, for example, peristaltic 

 locomotion is controlled by segmental reflexes. Progression of waves of 

 swelling and shrinking along the body involves reciprocal excitation and 

 inhibition of longitudinal and circular muscles in each segment in turn. 



Errant polychaetes have several forms of locomotion, crawling and 

 swimming. Fast creeping in Nereis involves the successive movement of 

 groups of from four to eight parapodia. Accompanying the wave of 

 parapodial beat, the longitudinal muscles of one side contract, while those 

 of the other side relax. The tips of the parapodia are applied to the ground 

 during the backward stroke, and the motive power for traction is supplied 

 by the longitudinal muscles. At the beginning of locomotion the ambula- 

 tory pattern spreads rapidly over the body in an antero-posterior direction ; 

 subsequently, the pattern is transmitted at a slow rate in an anterior direc- 

 tion, i.e. the direction of progression. It seems likely that the rhythmic 

 contractile waves producing peristaltic locomotion in polychaetes in- 

 volve neural activity of several kinds : the propagation and setting of a 



M.A. — 15 



