458 THE BIOLOGY OF MARINE ANIMALS 



fishes which depend upon particular sequential stimuli to carry through the 

 entire spawning process (25, 112). 



The very rigidity of much instinctive behaviour, particularly complex 

 instinctive acts, is related to the intricacy of the situations in which the 

 participants become involved. It seems as if the animal has no compre- 

 hension of situations as a whole, no knowledge of goals, and no method of 

 juggling and relating perceptive concepts. In the absence of such cerebral 

 equipment the completion of a complex behavioural act — say, sexual dis- 

 play, spawning and nest-guarding in a teleost — depends upon an obligatory 

 series of ordered responses to particular stimuli presented in definite 

 temporal and spatial patterns. Of the total available field of stimulation, 

 only some one entity may be utilized for triggering a reaction, e.g. the red 

 belly of the male stickleback which acts as sign stimulus for fighting among 

 males during the breeding season. Failure of one stimulus in a series may 

 block the chain of reactions, and appearance of a signal in a novel setting 

 may lead to a misdirected response. Innate pathways in the c.n.s. appear 

 channelled or preset for particular actions under particular conditions, 

 Under normal circumstances the system is such that fruition of endeavour 

 follows. 



Variability of Response. Animals generally show a certain amount of 

 variability in behaviour and response. Even among the lowest animals the 

 effect of a stimulus depends to some extent upon the creature's physio- 

 logical condition and recent history. Under repetitive stimulation of an 

 irritative nature, protozoans often respond by a series of negative avoiding- 

 reactions of various kinds. The repertoire of responses, however, is limited, 

 and the kinds of responses are simple and stereotyped in nature. 



Alteration of response with repetition of stimulus has been observed in 

 coelenterates and echinoderms, and the experiments serve to underline 

 certain differences between sensory adaptation and true learning. Responses 

 of actinian tentacles (Metridium, Stoichactis, etc.) change with continued 

 feeding, and at one time this result was attributed to loss of hunger. Further 

 analysis shows that the responses of the tentacles disappear even when the 

 animal is not permitted to swallow the food. Muscular fatigue is not 

 involved, since the tentacles can be caused to contract an equivalent num- 

 ber of times without reducing responsiveness. Furthermore, when the 

 tentacles on one side of the animal are plied with food, which is removed 

 before it is swallowed, those tentacles fatigue, whereas the tentacles of the 

 opposite side remain unaffected. An analogous situation exists in the 

 behaviour of certain ciliary fields, which reverse direction when subjected 

 to food and mechanical stimulation. When the animal is satiated with 

 food, ciliary reversal can still be evoked by continued application of food 

 but not by mechanical stimulation. The alterations in responses produced 

 by these experiments are due, not to fatigue of effectors or to changes in 

 the nerve-net, but to the onset of sensory adaptation or accommodation 

 (85, 116). 



Plasticity of response is little more advanced in echinoderms, as the 



