SENSES OF THE EARTH- WORM. 69 



tlie centre in various directions, and the precise i)atli cliosen 

 depends on some unknown- 

 action taking place in the 

 centre. The action of the 

 centre moreover may be 

 modified by efferent iin})ulses 

 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 Fio. Sl.-Diagram representing three nerve- 

 •^ ^ ^^ ^ centres and connections. Arrows represent 



out of elements similar to the possible direction of nerve-impulses, 

 reflex actions ^•^' °^^ afferent path ; e/, 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 wlmse 

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

 some reason to believe that the cerebral ganglia occupy a higlier 

 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. Exj^eriment has shown furtlier tluit 

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

 trollinfi^ action over those of the ventral chain bv means of im- 

 pulses sent backwards through the commissures, though tliis 

 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 iiigher animald 

 possess definite "sense-organs" which receive stimuH and thntw 

 into action the sensory nerves proceeding from tlieni. Ahlioiigh 

 the earthworm possesses the "senses" of tuueh, taste, sight, 

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

 the general integument covering the surface of the Itodv. and 



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

 ology. 



