Phases of Activity in the Sea-Anemone, Metridium Senile (L.), and Their Relation to External Stimuli

Abstract

1. In constant environmental conditions Metridium senile exhibits continual slow inherent activity. The pattern of this activity varies in character from time to time. These different patterns of activity have been termed ‘phases’. A particular phase may endure for hours on end and then rather quickly give place to another phase. 2. A change of phase may be initiated by certain stimuli. Ingestion of food initiates a sequence of phasic changes involving expansion of the disk and elongation, followed by swaying, parietal contraction, distension, defaecation and ‘shrivelling’. Each of these phases has its own pattern of activity. Not all may be exhibited by any one animal, or by the same animal at different times. A similar sequence of phases to those induced by solid food may be initiated by mere temporary exposure to filtered food solution. 3. In contrast with the direct responses, such as the retraction reflex, the stimulus does not directly maintain a phasic response; it merely initiates a new phase, and the activity pattern of this new phase is maintained long after (even hours after) the initiating stimulus. This relation of phase change to stimulus may be evident after electrical stimulation as well as after exposure to food or other stimuli. 4. Phase changes also differ from simple reflexes in that the threshold for their initiation varies enormously in different animals and in the same animal at different times. Also the threshold may sometimes be so low that the phase occurs spontaneously in the absence of evident external stimuli. Changes of threshold cannot be adequately accounted for by sensory adaptation. The stimulus apparently acts by releasing a complex activity pattern (the phase) which is so far independent of the stimulus that it may appear spontaneously. 5. Locomotion is another phasic activity. It is a complex co-ordinated activity pattern. It may be initiated by various stimuli. The threshold varies at different times and in different animals. It may take place spontaneously in the absence of evident external stimuli. The threshold of this phase is lowered after feeding and raised by illumination. 6. Alternating phases of expansion and contraction frequently occur in Metridium. Their relation to diurnal and other rhythms is discussed. Daily illumination can often initiate and control regular daily phases of contraction. This is true both of daylight and of periodic exposure to artificial light. As with other phase changes, the threshold varies greatly. In some cases each periodic illumination may only induce a brief temporary contraction and fail to control phase change. 7. In complete darkness and constant environmental conditions, alternating phases of expansion and contraction may still take place. These may be irregular, but sometimes they may assume a very rough rhythm. When present, this rhythm does not keep in step with previous daily stimuli, nor with current external changes of day or night, nor with other environmental rhythms. The unstimulated animal thus possesses an inherent tendency to alternating phase change which may approach a rhythm. It appears that periodic stimuli, such as daily illumination, act by ‘setting the pace’ of this inherent alternating phase change. 8. Different phases which involve the same groups of effectors may reinforce or may conflict with one another. 9. Our experiments show that continual and varying patterns of inherent activity play an important part in the behaviour of Metridium. The behaviour is not simply a succession of direct reflexes to stimuli acting on a passive animal. Such direct responses are, however, more easily observed because phasic activity is extremely slow. Phasic activities play an essential part in behaviour patterns such as food capture. They are often behaviouristically relevant to a future possible event rather than to a past stimulus; as when sweeping and swaying movements increase the chance of finding food. The relation of phases to the ‘physiological states’ of Jennings is noted.