. The biology of marine animals. Marine animals; Physiology, Comparative. 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
. The biology of marine animals. Marine animals; Physiology, Comparative. 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. 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. ). 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.
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