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Show 22 HABITS OF WORMS. CHAP. I. to act perhaps once out of half a dozen trials. The light was on one occasion concentrated on a worm lying beneath 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 ]amp or a candle invariably retreated into their burrows within from five to fifteen 1ninutes; 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 CHAP. I. THEIR SENSES. 23 different occasions might be explained, either by the degree of extension of their skin and its consequent transparency, or by some particular incidence of the light; but I could discover no such relation. One thing was manifest, namely, that when worms were employed in dragging leaves into their burrows o~ in eating them, and even during the. short 1ntervals whilst they rested from their. work, they either did not perceive the hght or were regardless of it; and this occurred even when the light was concentrated on .them through a large lens. So, again, whilst they are paired, they will remain for an hour or two out of their burrows, fnlly exposed to the morning light; but it appears fr?m wha.t Hoffmeister says that a light will occasiOnally cause paired individuals to separate. When a worm is suddenly illuminated and dashes like a rabbit into its burrow-to use the expression employed by a friend-we are at first led to look at the action as a reflex one. The irritation of the cerebral ganglia appears to cause certain muscles to contract in an inevitable manner, independently of the will |
