NERVOUS SYSTEM AND BEHAVIOUR 453 



likewise show a rhythmic pattern of activity, moving about most actively 

 at dawn, and lying quiescent beneath the sand at midday. The environ- 

 mental factor controlling this diurnal activity is light. Animals will main- 

 tain a persistent rhythm for several days in total darkness, but the rhythm 

 gradually disappears. Hunger modifies the vigour of this activity but does 

 not control its periodicity. There is apparently an inherent tendency to- 

 wards rhythmic behaviour, the timing of which is set by recurrence of day 

 and night (38, 71,79). 



Simple ascidians show a constant pattern of spontaneous activity which 

 in Phallusia takes the form of regular contractions of the siphons (squirt- 

 ing) at intervals of 6 to 9 min. The contractions of both siphons are 



Fig. 10.23. Spontaneous Squirting of Phallusia mammillata 



Records show movements of siphons of a deganglionated animal. Upper tracing, 

 branchial siphon; lower, atrial siphon. (From Hoyle (59).) 



synchronous, and the frequency may shift fairly quickly under constant 

 conditions to higher or lower rates usually twice or one-half the normal 

 rate. The pacemaker for this activity is situated in the body wall, either in 

 the musculature or in the nerve-net. The periodicity is not abolished by 

 deganglionation, which does, however, affect co-ordination, tonus and 

 reflex responses (Fig. 10.23). Spontaneous activity is also shown by isolated 

 muscle strips {Phallusia) and isolated siphons (Ciona, Styela). This activity 

 is intrinsic, as in sea anemones, and is sensitive to environmental changes, 

 increasing during starvation and declining in the presence of food (58, 59). 

 Among animals which possess well-defined ganglia and central nervous 

 systems it is sometimes possible to assign the regulation of spontaneous 

 activities to particular centres. The behaviour patterns of some polychaetes 

 are particularly interesting in this regard. Arenicola is a burrowing poly- 



