NERVOUS SYSTEM AND BEHAVIOUR 451 



ments, diurnal rhythmicity of pigment responses, etc. Some of the evidence 

 relating to the intrinsic regulation of such functions will now be examined. 



Rhythmic activity occurs in many kinds of cells. Among unicellular 

 algae and protozoa there are estuarine and littoral species which display 

 persistent rhythms related to tidal periodicity. The estuarine flagellate 

 Euglena Hmosa, for example, continues to perform periodic migrations 

 under constant laboratory conditions, in rhythm with natural tidal ex- 

 cursions (35). 



In higher animals there are tissues which show periodic or rhythmic 

 activity, e.g. the periodic beat of the myogenic heart, although such 

 activity is often under nervous control. Periodic or rhythmic activity is a 

 cellular characteristic, shared by nerve cells. 



Spontaneous Activity in Animals possessing Nerve-nets. Simply organized 

 animals having nervous systems organized as nerve-nets often show com- 

 plex behaviour patterns. In an apparently simple animal, such as the 

 jellyfish Amelia, the bell contracts rhythmically, executing periodic 

 swimming movements. Bouts of activity are interrupted by periods of 

 quiescence, and each outburst consists of a series of contractions variable 

 in frequency. The activity is due to rhythmic discharge from a series of 

 ganglionic pacemakers lying at the bases of the marginal tentaculocysts; 

 these pacemakers discharge into the nerve-net. Although spontaneous, 

 the activity of the pacemakers can be altered by external stimuli, leading 

 to modification of the response (84). 



Anemones such as Metridium display slow but continual muscular 

 movements. In the column, such activity consists of a sequence of regular 

 reciprocal contractions of parietal and circular muscles, and the behaviour 

 may be markedly rhythmic, with a period of about 10 min. Activities of 

 different parts of the body wall are usually co-ordinated, and contraction 

 of one part of the parietal musculature is followed by contraction of 

 others. It appears that co-ordination of movement takes place through one 

 part of the body wall acting as leader, other regions following this con- 

 traction with long delays. Activity of this nature is inherent in Metridium 

 and continues unaltered in the absence of external stimulation (Fig. 10.21). 

 The pattern of activity, however, changes from time to time, and particular 

 phases may be initiated by external stimuli such as food and light (Fig. 

 10.22). In complete darkness, and with other environmental conditions 

 maintained constant, alternating phasic activity may still show a rough 

 diurnal rhythm. It is concluded that inherent and spontaneous phasic 

 activity is influenced by periodic stimulation, such as diurnal illumination, 

 which determines the period of rhythmicity (8, 9, 80, 82). 



Phasic Activity in Animals with Central Nervous Systems. Spontaneous 

 phasic activity appears at all levels of central nervous organization, in 

 manifold aspects of internal functioning and external behaviour. Periodic 

 discharge of ganglionic pacemakers controlling circulation and respiration 

 are cogent instances of the former. Crabs and molluscs, under laboratory 

 conditions, show periodic fluctuations of oxygen-consumption, in sym- 



