450 THE BIOLOGY OF MARINE ANIMALS 



reflex pattern along the length of the c.n.s. ; potentiation by chain reflexes 

 involving tactile or muscular receptors; and activation of antagonistic 

 central arcs controlling the tonus of reciprocal segmental muscles (39, 

 40). 



The distinctive reflex responses of certain polychaetes have been 

 analysed sufficiently to reveal some part of the neural pathways involved 

 in their regulation. Some examples are: startle reactions of nereids and 

 sabellids to tactile stimuli (p. 434) ; the response of Protula (a serpulid) 

 to sudden decrease in light intensity; the burrowing of Branchiomma. 

 In Protula a patch of primary photoreceptors in the head connects with 

 the giant axons, which are the penultimate neural units on the efferent 

 side of the reflex arc. Branchiomma is positively geotropic, and regulates 

 the direction of burrowing by means of information received from stato- 

 cysts and tactile receptors. Statocysts, located in the head, send impulses 

 through the length of the nerve cord, and it is on the basis of this informa- 

 tion that particular muscles are thrown into activity in posterior segments, 

 so that the animal always burrows with its tail directed downwards, 

 whatever the position of the statocysts (15, 24). 



Reflex responses in molluscs and arthropods are mediated almost 

 exclusively by central ganglia. The extremely interesting condition occurr- 

 ing in animals of these two phyla, whereby muscular activity is graded 

 peripherally by excitatory and inhibitory fibres, is described in Chapter 9. 

 Locomotory movements in decapod Crustacea are reflexly controlled, the 

 legs moving in a definite pattern relative to each other. However, when one 

 leg is removed, changes in the locomotory pattern occur at once, and the 

 remaining limbs assume new movement-sequences. In explanation it 

 appears that the c.n.s. must contain alternative nervous settings for differ- 

 ent ambulatory patterns, which can be brought into operation as required 

 by particular external conditions. 



Stimulation of individual large fibres in the crustacean c.n.s. often brings 

 about widespread contractions or reactions. A single impulse in the medial 

 giant fibres, in particular, produces motor reactions throughout the animal. 

 In another instance a whole reflex pattern (the defensive reflex) is obtained 

 by stimulation of a single fibre in the circumoesophageal connective. 

 Neurones are notably sparse in the crustacean nervous system, and com- 

 plex reflexes are evoked by a narrow range of stimuli and mediated on the 

 efferent side by greatly restricted motor pathways. The stereotypy of 

 crustacean responses is in contrast to the greater plasticity of much 

 vertebrate behaviour, founded on a richer and more varied basis of 

 central neural organization (125). 



Spontaneous Activity. Spontaneous activity is a common feature of 

 animal behaviour, and the question arises as to what extent this kind of 

 activity is entirely intrinsic in origin and to what extent it is influenced by 

 environmental agencies. Spontaneous cyclic or rhythmic functions are 

 encountered at all levels of organization, and various instances have been 

 noted on other pages, e.g. rhythmic beating of hearts, respiratory move- 



