ANIMAL BEHAVIOUR 187 



In typical and simple cases, a reflex action involves (1) the 

 receptor of a stimulus the sensory or perceptory nerve-cell 

 from which impulses pass in to the central nervous system ; 

 (2) a ' motor ' nerve-cell which connects the central nervous 

 system with a muscle or a gland; and (3) between these 

 two a l communicating ', or internuncial, or * associative ' 

 nerve-cell connecting them within the nervous system. The 

 receptor neurone has its cell-body outside of the nerve-centre ; 

 the motor neurone, with its cell-body within the nerve-centre, 

 sends a nerve-fibre to some peripheral effector organ; the 

 associative neurone connects the two others. Thus is formed 

 a ' reflex arc ', the functional unit of the nervous system. In 

 most cases the arrangements are more complex and several 

 ' reflex arcs ' become interlinked. But the point is that 

 reflex actions do not require individual correlation; that is 

 pre-established. Yet it is important not to think of reflexes 

 too simply. 



Combination of Reflexes in Unified Behaviour. The per- 

 fection of reflexes is well illustrated in the behaviour of 

 a sea-urchin, which has no nerve-ganglia. Its test is cov- 

 ered with mobile spines and snapping blades (pedicellariae) 

 which react in definite ways to definite stimuli and have an 

 astonishing independence. For a single spine or pedicellaria 

 on an isolated fragment of shell reacts very much as usual. 

 In the uninjured creature the spines and other structures are 

 all connected by a nervous network on the surface of the 

 shell, and they act harmoniously, working into one another's 

 hands, securing effective defence and locomotion. According 

 to von Uexkiill the sea-urchin is a " republic of reflexes ". 

 " The separate reflex arcs are so constituted and so put to- 

 gether that the simultaneous but independent course of the 

 reflexes in response to an outer stimulus produces a definite 



