NERVOUS SYSTEMS OF INVERTEBRATES 



it. That this is so in the common earth-worm we know from 

 the fact that the mole stores up these unfortunate creatures as 

 a sort of living larder, having pre- 

 viously bitten off the front ends of 

 their bodies, and consequently re- 

 moved such brains as they possess. 

 This does not destroy life, but pre- 

 vents the victims from crawling away. 

 The ventral nerve-cord is subordi- 

 nate to the brain, but exerts a con- 

 siderable amount of independent con- 

 trol, each pair of ganglia dominating 

 the ring or segment to which it be- 

 longs. To these collections of nerve- 

 cells are due what are technically 

 known as reflex actions, which are 

 quite independent of will. We may 

 instructively consider one common 

 sort of reflex action which manifests 

 itself in muscular movement. If the 

 skin of one of the segments is stimu- 

 lated mechanically, chemically, or 

 otherwise, some amount of contrac- 

 tion in the muscle of the body-wall immediately follows. For 

 the performance of this or any other reflex action three nervous 

 elements are requisite: (i) a nerve-centre consisting of one or 

 more, usually of several, nerve-cells, which 

 in the latter case co-operate with one an- 

 other; (2) one or more nerve-fibres consti- 

 tuting an afferent tract carrying impulses to 

 the nerve-centre from sensitive ectodermal 

 cells which have been acted upon by the 

 mechanical, chemical, or other stimulus; 

 and (3) one or more nerve-fibres forming sense-or ga n ; ^., afferent nerve; 



\J f N.C., nerve-centre; eff.n., efferent 



an efferent tract carrying" impulses from nerve. Direction of nerve -impulses 



. J . J indicated by arrows. 



the nerve-centre to the executive structures 



which perform the reflex action, these being muscle-fibres in the 

 case supposed (fig. ion). Even in ourselves many actions are 

 of reflex nature, e.g. the involuntary withdrawal of the hand from 

 a red-hot substance as described on an earlier page (p. 2). 



Fig. roio. Diagram of part of the Ventral 

 Cord of an Earth- Worm, showing a few Neurons, 

 enlarged. The two arrows (on the right) indicate 

 the direction of nerve-impulses, i, Nerve-roots; 

 2, afferent nerve-fibres; 3, efferent nerve-fibres; 

 4, a neuron, of which the branches extend through 

 ents 



N.O. 



