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- 

 contr oiled and guided, and 

 how even the most compli- 

 cated forms of nervous ac- 

 tivity may be Compounded FlG ' K--**"M representing three nerve. 

 * * centres and connections. Arrows represent 



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

 is . a/, 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 

 system is distinguished from the nervous system as a matter of 

 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 Phy 

 ology. 



