NERVOUS SYSTEMS OF INVERTEBRATES 9 
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 
Fig. roro.—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. 1, Nerve-roots: 
z, afferent nerve-fibres; 3, efferent nerve-fibres: 
4, a neuron, of which the branches extend through 
three segments 
immediately follows. For 
the performance of this or any other reflex action three nervous 
elements are requisite: (1) 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 Zo 
the nerve-centre from sensitive ectodermal 
cells which have been acted upon by the Muscle — 
mechanical, chemical, or other stimulus; 
Fig. 1or1.—Diagram of a Simple 
Reflex Action. S., Stimulus; S.O., 
and (3) one or more nerve-fibres forming sense-organ; affiz., afferent nerve; 
NV.C., nerve-centre; ef., efferent 
an efferent tract carrying impulses from nerve. Direction of nerve-impulses 
the nerve-centre to the executive structures 
indicated by arrows. 
which perform the reflex action, these being muscle-fibres in the 
case supposed (fig. ror). 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). 
