SENSES OF THE EARTH- WORM. 69 



the centre in various directions, and the precise path chosen 

 depends on some unknown- 

 action taking place in the 

 centre. The action of the 

 centre moreover may be 

 modified by efferent impulses 

 arriving from other centres, 

 and thus we can dimly per- 

 ceive how reflexes may be- 

 cont rolled and guided, and 

 how even the most compli- 

 cated forms of nervous ac- 

 tivity may be Compounded FlG * ^.-Diagram representing three nerve- 

 > * centres and connections. Arrows represent 



Ollt of elements similar to the possible direction of nerve-impulses. 

 M - 4 af, one afferent path; ef, one efferent path. 



There is reason to believe that in the earthworm each ven- 

 tral ganglion presides over the somite to which it belongs, and 

 is probably in the main a collection of reflex centres from whose 

 action the element of consciousness is absent. But there is also 

 some reason to believe that the cerebral ganglia occupy a higher 

 position, since they probably receive the nerves of sight, taste, 

 and smell, besides those of touch, while the ventral ganglia re- 

 ceive only those of touch. Experiment has shown further that 

 the cerebral ganglia exercise to a certain limited extent a con- 

 trolling action over those of the ventral chain by means of im- 

 pulses sent backwards through the commissures, though this 

 action is far less conspicuous here than in higher metameric ani- 

 mals such as the insects.* 



The Sensitive System. (Organs of Sense.) The sensitive 



svstem is distinguished from the nervous system as a matter of 



t/o n 



convenience of description, since most of the higher animals 

 possess definite " sense-organs ' ' which receive stimuli and throw 

 into action the sensory nerves proceeding from them.. Although 

 the earthworm possesses the "senses' of touch, taste, sight, 

 and smell, it has no special organs for these senses apart from 

 the general integument covering the surface of the body, and 



* For a fuller discussion the student is referred to special works on Physi- 

 ology. 



