22 HABITS OF WORMS. CHAP. T. 



to act perhaps once out of half a dozen trials. 

 The light was on one occasion concentrated 

 6>n a worm lying beneatli water in a saucer, 

 and it instantly withdrew into its burrow. 

 In all cases the duration of the light, unless 

 extremely feeble, made a great difference in 

 the result; for worms left exposed before a 

 paraffin lamp or a candle invariably retreated 

 into their burrows within from five to fifteen 

 minutes ; and if in the evening the pots were 

 illuminated before the worms had come out of 

 their burrows, they failed to appear. 



From the foregoing facts it is evident that 

 light affects worms by its intensity and by 

 its duration. It is only the anterior 

 extremity of the body, where the cerebral 

 ganglia lie, which is affected by light, as 

 Hoffmeister asserts, and as I observed on 

 many occasions. If this part is shaded, other 

 parts of the body may be fully illuminated, 

 and no effect will be produced. As these 

 animals have no eyes, we must suppose that 

 the light passes through their skins, and in 

 some manner excites their cerebral ganglia. 

 It appeared at first probable that the dif- 

 ferent manner in which they were affected on 



